Jason: The swap is realtively staight forward,but there are a few things you have to do
1) Change yoke on your driveshaft. The FMX uses a 31 spline and the C-4 uses a 28 spline. Just try to get one that uses the same u-joint size to eliminate the need to use a special u-joint with two different widths across the caps. C-4 and FMX are the same length, so you don't need to change the driveshaft.
2) Tranny lines will have to be re-bent or new ones fabbed up, as they attach to the trans in different locations but still on the passenger side.
3) You may have to use a different flexplate. I can't remember whether the C-6 and FMX are the same, or the C-4 and FMX.
4) Your crossmember, starter, shifter and mount should all work fine. The C-4 has the neutral safety switch on the trans, yours should be in the shifter. Should not be a problem, just keep using the one in the shifter. Make sure you get a C-4 for a floor shift car, as the lever on the side of the tranny is different, and you have to remove the valve body to change it.
I'm probably forgetting something here, but I'm sure others will refine what I've started here.
Well I believe that covers almost all but I put a C-4 in my 73XR7 and had to fabricate a piece to allow me to use the same mounting location.My C-4 was a couple inches longer at least for the cross member mount than a FMX. Now the flywheel I don't believe will have to be changed as you already have a 351. I'm sure their are some other that will fill in any gaps.
I swapped out a fmx out of a 69 torino and put in a c4, we had to change the flywheel to the one with the c4 so that the starter would engage and then start the car. Good luck, pretty straight forward.
I've been told I should probably get the C4 rebuilt to handle the 351W. Would it be cheaper to find a junkyard trans and have it rebuilt, or order one from TCI ($750, supposed to be able to withstand up to 450 hp)?