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Ok here it is in a large nut shell. I had a 41 1 1/2 fire truck with a 216 and a stock 4spd. Built a dump truck out of it used a 54 235, 51 bell housing, 54 4spd. Switched from 6" 20's to more modern 19.5's. Removed dump bed shed close to 2000 lbs of iron, shortened the long wheelbase frame 22". Currently have 6.11 rear end. I have purchased a rear end out of a 70's motorhome, it had a 454 with an automatic. I don't know what gear ratio it is, it was cheap with ten lugs it fits my wheels. The pig is 12" across where mine is 16" so it has to be more highway friendly than what I have. I also found a 5spd out of a 1960 60 series truck. It bolts up to my bell, it has an e brake with the same yoke pattern. Basically my plan is to create a really heavy duty pickup with dual wheels. I finally found some great fenders perfect for my set up. I am using an 8' 46 GMC bed. The Truck is basically going to weigh the same as pickup on top but have the weight of a short wheel base 1 1/2 ton underneath. So what would you use the 4 spd or the 5? The 5 will require modifications to the floor pan again it seems to be a bit taller. I am trying to get this beast to go 50-55mph without sounding like it is going to explode. Grigg thinks the rear is probably a 4.56. I was hoping for a 4.10. I got the 5spd just in case I needed help getting it moving. I was trying to find out what gear ratios the 5 speed has I assume its final is the same as the 4sd. But I was curious what 1st 2nd & 3rd gear ratios were like. I am using a fresh 235 but I have a 261 in reserve if needed. I know I don't have much power to work with but I really don't plan to haul much weight with it either. Suggestions would be helpful. Thanks Joe N.

I think you really want to know what the gears in that replacement rear are. I have two rears from similar sources that are 4.11 and 3.70.

The 3.70 rear is under a '49 1-ton that I got from the guy who put the rear under it. He said with those gears and the 216, with an SM420, the truck was not capable of moving down the road acceptably.

Your truck will be heavier than the 1-ton, but that only balances the 235 in my mind. If your gears are 4.11 or higher you may find it doesn't have nearly enough power.

I don't feel you can answer the transmission question without first knowing the rear gears.

I think 4.56 would be as high as you could pull with the 19.5 tires. Some of those 5 speeds were 5th over but most were direct in 5th. It is a huge heavy trans. I would stick with the 4 speed.

I had my 52 1 ton pickup setup with 4.56 gears, 8R19.5 tires, SM420 transmission, and a 1955 model 235 engine, truck weighed 4,400 lb empty.
I drove it round the country, from dead flat to big mountains, and mostly unloaded although we did have several hundred pounds of gear/tools and extra fuel. Speed was limited by the slow gears 98% of the time, on some of the really big mountains I ran out of power and would down shift. We traveled between 2,200 rpm and 2,500 rpm, at between 50-55 mph, occasionally faster. I would have really liked 4.11 gears, and probably even 3.73, although with the 3.73 gears a direct drive 5 speed with a close 4th and 5th ratio may have been helpful.

If you know you will keep the rear axle you have found, figure out what gear ratio it is, and give it a try, if you feel like it needs faster or slower gears, or a different transmission, then take that step once you get there.

Make a reasonable guess with what you have already, try it, and recalculate and swap things later.

Just an example; I assembled a diesel truck and calculated that 3.73 gears, 0.80 OD, and 38" tires would be just perfect, once I got it together I drove it on an 8,000 mile trip (two weeks), and not long into the trip came to the conclusion that 3.54 gears would have been much better than the 3.73 I used.
Point is you don't really know till you try it.

Grigg

Joe, that sounds like a great project. Did you know that General Motors actually made a 1-1/2 ton dually pickup in 1941? It used a 134-1/2" wheelbase and 9' bed.

As for your drivetrain dilemma.... You're not going to use that 70's motorhome rear end without replacing the gear oil are you? Well, just pop that cover off and count the gear teeth when you change the oil, and that way you can know for sure what you've got to work with. (assuming it's the Dana)

You asked about top speed and transmissions. Both of your transmissions are very-very likely 1:1 in top gear, so top speed is the same. Since the four-speed is already in the truck, I would try that first. Also, if it's a '54 tranny, then it will be fully synchronized, unlike the double-clutcher your truck came with, so that's good. If you then find you don't have enough power to get moving, then maybe your gears are too fast or you could consider switching to the 5 speed to give you smaller steps. Since you're running empty most of the time, the four speed will probably work fine---but it all depends on your gears.

I'm curious about your rear fenders. What did you find to work? The dually rear fenders are very rare--in fact I had to buy a whole 'nother truck to get a pair since mine were missing.

--Bob

Here is deal, the camper has kind of taken to nesting, the skirt is almost touching the ground. It is very hard to get under and dark. The tag on the pan has rusted off. The chassis is GMC. The guy I am getting from says if I don't like it after he gets it out I don't have to take it. He likes me I sold him some decent sheet metal for his 46, cheap! Anyway I don't want you guys to think I am crazy but when you have 6.11 gears 30mph is comfortable and 40mph means your yelling to talk, 50mph is down right crazy. My SM420 was a good upgrade, but this new, old 5 spd was going to be scraped so I bought it from the scrap dealer. $150 if I took it out of the wrecker, I knew the guy who had the wrecker and he said it worked fine. It is one inch longer and about 3/4 of an inch taller than my SM420. It has a e brake on the rear as well. Everything bolts up. $150 is three tanks of gas in my car no big loss. It has a final 1 to 1 but I was thinking if the new rear end was not quite right this would give me more options. The fenders came off the wrecker as well they are a perfect fit around my tires. They originally came off an old mail truck from the 40's. They are too narrow but that can be solved easily. Once I decide if I am running duals or singles. I might make them ajustable. I am going to go with Griggs quote of Mr.Maudslay, Get a clear notion of the goal! I believe this problem can be solved.
P.S.Bob I also have a set of dual fenders from off a 42 GMC fire truck they are for the bigger 20" inch tires. Need some?