Re-post of a prior post...
We don't know what ratio of rear end you have at this point, and we don't know what transmission gearing you have. So, let's look at one option. If you can find a 3.07 or 3.08 rear end and a T5 with these gear ratios:
1st gear---4.03
2nd gear---2.37
3rd gear---1.49
4th gear---1.00
5th gear---.86
(There were lots of these out there. Mostly they were used in S10 trucks or Isuzus with either 4 or 6 cylinder engines. 1982-1984 as I recall.)
The rear end with a 3.07 or 3.08 ratio.... common in 12 bolt rear ends and some used in late 60s or early 70s pickups were the right width. Your driveshaft may or may not work...you'll have to see about that. The T5 is roughly 25 inches long from where it bolts to the bell housing to the end of it.
Ok, if you find these two combinations, here's what you'll have as your final gear ratios:
1st gear---4.03 x 3.08 = 12.41
2nd gear---2.37 x 3.08 = 7.30
3rd gear---1.49 x 3.08 = 4.59
4th gear---1.00 x 3.08 = 3.08
5th gear---.86 x 3.08 = 2.65
And these final ratios will go up/down based on the diameter of your wheels and tires.
So if you still want to be able to put 1/2 ton of "stuff" in the bed and drive like a pickup should drive, the above combination will work great for you. It is very close to the ratios the original GM engineers wanted in there, so for the first 3 speeds, you'll feel like nothing has changed except your first gear will be synchronized. There are other ways to skin the cat and I'll mention one of those in a moment. Then in 4th gear, you get your first overdrive gear (in my case it was a 25% reduction from the stock 4.11 rear that was originally in the truck). Then on nice flat or slightly hilly roads, in 5th gear you get your second overdrive gear (a 14% drop) and you'll be cruising (60 or 65 mph) at 2000 to 2200 rpm depending on your tire size, etc. With a 235 I really don't believe you want to cruise at less rpm than that. Torque maxes out around 2000 and that's a good spot for you. You don't want the six to be lugging.
If you have a 3.55 rear end, you will do better with a T5 that has the following gear ratios:
1st gear---3.50 (x3.55 = 12.425)
2nd gear---2.14 (x3.55 = 7.597
3rd gear---1.39 (x3.55 = 4.93)
4th gear---1 (3.55)
5th gear---.73 (x3.55 = 2.59)
You can see the final gear ratios change a bit from the 3.08 scenario but stay in the same ballpark. The 5th gear ratio is a bit lower than I'd like, but a slightly smaller tire diameter would solve that if needed. The bad news is this transmission might be harder to find. As I recall this one was only used in 85 and 86 minivans with V6 engines. I think there may have been more T5s made with the 4.03 first gear scheme, but I could be wrong on that.
Or, you may be able to change your rear end gearing to 3.38 and go with one of the T5s with the 3.76 first gear...as I said different ways to get to the same spot. You just want to be very mindful of the first gear overall ratio...some would call this your SLR or Starting Line Ratio and other people might call it your takeoff ratio. With a pickup and an inline 6 I believe you need that to be at the minimum 11 or above. Otherwise when you start off there won't be anyone home under the hood and your clutch won't last long.
Important note: you may have to settle for a T5 with an electronic speedo output. Those with mechanical output are getting hard to find and costly. If you do, don't worry. There are ways to overcome this. If you look in the electrical bay section of this forum, you'll see where I (and a couple of other people) have been doing work and explaining how we're doing this. And it is possible to have the electronic speedo look pretty much like the original. So, you do have choices. Good luck no matter which direction you go.
