I have a 1980 f-150 with a 302 and c-6 4x4
ive found a 1981 ltd with a 351w that imwanting to put in my truck
what transmission is that car and should i use it instead of my c-6?

 

use the c6 it is a bolt up and it is by far the stronger transmission.

 

not to mention the fact that the car tranny won't take well to mounting a transfer case to it so you don't really have much of a choice you basically have to use the truck's c6. it is a good choice though so don't worry.
one thing though make certain that you use the flexplate from the 351 if you're tourqe converter will not fit that flexplate then buy one for a 351W that will fit. do not use the 302 torque converter as that year has a different balance then yours does.