If you have a 350 Chevy engine and the transmission is from a Chevy then it will bolt up with no problem. The th-400 was also made with a BOP (buick olds pontiac) bolt patten along with many others.
The output shaft has a different spline configuration than the th-350 so a different yoke will be required.
Depending on the tailshaft length, you may need a different length of driveshaft.
If the transmission is from a TRUCK and has a provision for a bolted on yoke it will still work but requires a specially machined yoke and a longer driveshaft.
The th-350 has machanical kick down linkage, the th-400 uses an electric kick down triggered by a switch near the gas pedal.
use a 375 turbo,it has 400 internals & a 350 turbo tailshaft. make sure your tail housing lenth is the same as your old trans & your existing driveshaft should work MAKE sure trans is a chev. pattern! they bolt up to chev 6cyls. to big blocks! most chevs. already have the kick down bracket configured into the gas pedal bracket,all you need to do is bolt in swich,run power into switch & a wire down to trans.this all works!..i've done this a few times