Is this a 2wd or a 4wd? All above is true. Lengths are different, Tcase will be different, Driveshaft Yokes will be different and you'll have to add the Detent Cable bracket to the intake manifold and provision for the detent cable to the carb.
However, I still feel a Th350 to a Th400 is worthwhile. If you're not running a big block or doing heavy towing, the Th350 is a good choice for mild to moderate work duty. Th400 is what you'd want for moderate to severe duty. As far as driveshaft mod, don't mod the driveshaft. The Th350 driveshafts are out there for 2wd in U Pull it yards. A truck with a 700r4 driveshaft will work too provided your Th350 is a 9in tailshaft which is the OE size for squarebody trucks. So any 2wd truck with a Th350 or 700r4 will fit, truck for truck and have the correct yoke on it since Th350 and 700r4 have the same output splines. What I mean is, half ton long bed for half ton long bed. A shortbed driveshaft for a longbed truck is going to be to short of course. Also, try to match up the donor driveshaft with the same rear end incase there is a difference in length of a 10bolt or a 12bolt. I think there is a little difference there. Finding a good used one will be much cheaper than getting yours modified to work. I'd put new U joints in it though if they are OEM.
The Th350 might give you a hair better mpg, will for sure increase the horsepower to the wheels due to less paratitic loss thru the tranmission. Th350 is also lighter weight, and a tad better 1st gear. If your Th400 converter is still good, it'll also fit the Th350. You can also use the same flexplate and shift linkage. Crossmember will be in a different location too. Guessing about 2in closer to the motor for the Th350. Reason being, Th350 has the mount on the case, Th400 has the mount on the tailshaft so it puts the crossmember in a different location.
