I used a cast iron T10 behind my 270 in my first 59 gmc. Seemed to work pretty well. An sm420 wouldn't be a bad choice either. That would be the stock granny low transmission that was available for these trucks. You would need to put a carrier bearing and a 2 piece driveshaft for the sm420 to work.
Saginaw 4 speeds, muncie 4 speeds including the m21 and m22 will all bolt up to your bellhousing. Some may require a new clutch as the input splines may vary.
I didn't have to cut the driveline to put the t10 in, but I may have had to change the slip yoke on the front of the driveshaft. That was 15 years ago, so I don't remember all the details.
My current 59 gmc 1/2 ton longbed has a 59 pontiac v8 in it with a NV4500 5 speed. The original rearend was replaced by a previous owner with a chevy rearend with 3.90 gears. My v8 ate that rear end up so I am now running a 9 inch with 3.70 posi gears.
Anyway, Overdrive transmissions work well with rear gears in the 3.73-4.56 range. Direct drive transmissions work well with gears in the 2.73- 3.55 range.
3.08 gears are already on the economy side, so I would not go any taller. Taller gears would make it harder to get the truck moving. The 3.08 gears are absolutely useless for towing.
3.90 gears will have better acceleration than 3.08 gears, but freeway speeds won't be as high or comfortable... unless you have an OD transmission. Typically 3.9 gears with OD will give you a final drive of about 2.85:1 which is slightly taller than the 3.08:1 you have now. So 3.90 gears with an OD transmission will give you better acceleration, lower cruising rpms, and higher fuel economy than 3.08 gears with a direct drive transmission.
Use one of the many online gear calculators that give you rpm at any speed and play around with the numbers.
It is also important to take into consideration what the first gear ratio of the different transmissions are, or 2nd gear if you have a granny transmission. Anyway If you take the ratio of first gear and multiply it with the rearend ratio, This gives you a "Launch number". Typically a higher the Launch number would make it easier to start. Example a launch number of 6 will be slower off the line than a launch number of 12.
I know this is a lot of math... but it is important to ballance the performance with the economy desired. You will find that there are many tradeoffs here. If you want economy, you sacrifice acceleration. If you want acceleration, you sacrifice economy. If you want both economy and acceleration, you sacrifice a big chunk of money to get a new transmission and a new rearend.
I hope this information helps. I'm not trying to deflate your enthusiasm. I'm just trying to give yo the tools to chose what is best for you. By the way... I am by no means the expert. Others have ran the s10 transmissions with taller gears than 3.08 and liked the way they performed.
John
