I had a 98 3.9L magnum V6, and everything on the front end of the motor was identical to the V8 trucks. that stuff should all swap over--including the A/C (if you find the right lines because the motor puts them closer to the front on the V8 trucks) but the accessories them selves are all the same, the starter is the same, motor mounts should be the same, and if you're using a 318 neutral balanced motor, the front dampner is the same. can't use the rear flex plate or flywheel though, because the V6 only has 6 windows cut into it for the crank position sensor, the V8 needs...well...8.

all the EFI sensors should be the same though, MAP, IAT, TPS, CPS, Cam sensor, Water temp, etc.

Throttle cables will be different, unless you put the V6 TB on the V8. not a good idea, since it's smaller. but they use different linkage, so you will need a new throttle cable.