The Stovebolt Site Forums The Shop Area Driveline rear end/tranny reccomendations for 51 3600 flatbed

looking for some input before I go any further in this thing. I am hoping to use this truck for relatively light hauling and delivery, right now I have installed a rebuilt 283 and upgraded to front disc brakes, in order to do this i swapped the front axle to a half ton of a similar year truck and bought a 6 lug power disc brake kit from cpp. just drained the oil from the original tranny and a fair amount of water came out followed by some really gunky oil, flushed it several times then refilled with oil there is a dragging noise now whenever the clutch pedal isn't compressed even in neutral. before I go into trying to fix or replace it want to get some input on which way to go considering that I will probably end up wanting to swap out the rear end in order to get up to 65-70mph. have read about people using an s10 rear and tranny but this would leave me with 6 lug front and 5 rear. if I put in a 1/2 ton rear and tranny would I get up to the speeds I want? would be great to keep the 3/4 ton capacity but mostly want something that will get me down the road safe and reliably
thanks for your help this site has been a real help
aubrey

A 3 speed overdrive works well with the rear end you have now. There are a several on eBay right now including a Saginaw which is a very strong transmission. I suppose you are willing to put up with having different bolt patterns in front and rear.

You need to figure out what gear ratio will work best for you. Google "rpm calculator" and play around with some variables.

You have a lot of options for a six lug rearend because of your truck being a flatbed, as width is not as much of a concern with no fenders.

I would say your 3/4 ton capacity is already diminished due to your 6 lug front axle.

I have a semi-float 14 bolt axle from a 2003 van sitting here in the garage. I'm surprised at how stout it is built. There's a ton of support ribbing on the center section. Not to mention it has disc brakes. I know they had the 14 bolt in 6 and 8 lugs versions. So, that's my recommendation 14 bolt semi float with 6 lugs. A good place to look for the disc brake rearends are the 03-current 2500 vans.
If you're not concerned about rear discs, then just about any 1/2 ton 4x4 GM truck will give you a good 6 lug donor rearend.
Your 3/4 ton truck is about 800-1000 lbs lighter than any modern 1/2 ton, so there's plenty meat left on the bone even with a 1/2 ton rearend.
As far as trannies go. A 700R4/4L60 will give you a good trans. with overdrive. A turbo 350 will also do the trick behind a 283 and be super reliable also.

sorry for the delay in getting back appreciate the input, if I go with the 6 lug rear out of a van would I still be abvle to use the Saginaw? I think a standard with overdrive sounds like a solid idea but wanted to know if the shifter lines up in the same place and also what drive shaft would link them up. thanks again
aubrey

Shouldn't be a problem using a different rearend. No trannie swap required. You'll have to find the correct U-joint to make it all work though. Of course you'll have some cutting and welding time installing new spring and shock mounts. Just food for thought, going with a 3.42 rearend drops your RPM's 700-800 vs. the stock 4.57 gears. A 3.73 will drop the RPMS about 500. Is an overdrive really required at that point?

I have a 1950 3100 with a 1966 Saginaw 3 speed behind a 1953 235. The 12 bolt rear end is out of a 1973 truck with six lug. I don't know what the rear end gear ratio is, but I can tell you that I can keep up with traffic on the interstate with some gas pedal leftover.

sticking with the original transmission would sure make it easier, anybody know what the gearing was on a 51 3600 transmission?
I started looking at some van rear ends online what ratio did the 14 bolt semifloat whitedog mentioned come in?

also how hard will it be to find a replacement stock 3600 vs rebuilding one that was full of water. thanks

"anybody know what the gearing was on a 51 3600 transmission?"

first 7.05:1
2nd 3.57:1
3rd 1.70:1
4rth 1:1

sorry meant to say "stock 3600 transmission", also what can I check to be sure that the one I have under my truck is the one it came with?

sent that be fore I saw your reply bbsd, thanks for the numbers

ran that thru a rpm calculator using the 3.42 rear end option and came up with 2780 @ 70 in fourth, is that going to be too high with a 283?