Hey I am Brandon, I have a 250 Straight 6. It has a 3 speed that was on the tree and now converted to the floor. I would like to replace the trans with an automatic. The problem is I have no idea what kinds of transmissions I can put behind the 250. Any ideas?
Any of the Chevy transmissions will bolt to it, like the TH350, TH400, Powerglide, 700R4, 200R4 (last two have overdrive). It uses the same bolt pattern as the small and big-block Chevys.
TH350 or TH400, 700R4. Why do you want to go backwards with an automatic? Stay with a manual. Use a SM465 4 speed if you need to haul/tow or a T5 for a regular driver.
250/292 I-6 should share bellhousings. Depending upon your trans choice, you will also likely need a driveshaft. 
I just bought a 230 straight 6 with a 4-speed and am planning on running the 5-speed with overdrive from an S10 behind it. Pretty much any Chevy transmission from a car newer than the mid 60s will bolt up to the engine.
Gearheadotaku wrote: TH350 or TH400, 700R4. Why do you want to go backwards with an automatic? Stay with a manual. Use a SM465 4 speed if you need to haul/tow or a T5 for a regular driver.
+1.
Gearheadotaku wrote: TH350 or TH400, 700R4. Why do you want to go backwards with an automatic? Stay with a manual. Use a SM465 4 speed if you need to haul/tow or a T5 for a regular driver.
I like my manual, but where I live and drive there is a bunch of hills and traffic and it is more of a pain than what its worth. How hard would it be to get a TH350 in there, also would i have to change out my drive shaft with the TH350?
In reply to Gotta_Go_Fast:
Transmissions have all sorts of tail lenths and output spline counts.....its hard to say. The summit "Build your own driveshaft" kit is probably the cheapest way unless you find an automatic truck like yours to pull the parts from.
Don't forget the flex plate and torque convertor.
Just sayin'
a short shaft TH350 is an almost direct swap- you only need to change to a flexplate in place of the flywheel, and those are like $35 brand new. the trans mount will be in the same place, and your driveshaft will slide right in. this is the easiest option.
a short shaft TH400 is also an option, but you will need to shorten the driveshaft and get a different yoke.
if you want an overdrive, a 200-4r is also a direct bolt in, but you will have to slide the trans crossmember back.
a 700r4 is also an easy install with the crossmember slid back just like the 200-4r or TH400, but the driveshaft will have to be shortened a few inches (like the TH400), which is something you can do in your garage if you have a welder.
but why would you want to ruin the truck by going with an auto- it's got a Saginaw 3 speed in it now, so a 4 speed Saginaw is a direct bolt in affair- literally a half hour to do the whole swap (about 10 bolts total, including the driveshaft)if you take your time. you'll spend more time cutting the hole in the floor for the shifter to poke thru than the rest of the swap, and the extra gear makes it easier to find the right gear for every situation...
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