'69 Barracuda T56 swap, floor, cross member
spacing down the k-frame is going to play havoc on a bunch of things such as:
-Camber curve
-Steering column to box alignment
-torsion bar alignment (you may also be putting them into bending)
-extra stress on LCA bushings which are already a weak point on theser cars
The floor is already being modded. At least with this way, you can get the driveline angles right and also not mess the suspension or anything else up in the process. Changing the heat ducts I think is a very doable mod afterwards.
I've already got 5 years in on mine where I modded the floor but since I don't have an A/C box it was no issue. I also have the Quicktime bellhousing which is probably a little better for clearance, but still its not going to be so bad.