Here's a couple pics...
Revell '64-'65 on the left, AMT '72 GMC on the right:
As you can see, wheelbase is IDENTICAL.
And here's the Revell chassis inserted (easily) into the GMC cab, NO TRIMMING here. I haven't assembled one like this yet, but I foresee no problems, just a nip & a tuck here and there, as with any kitbash project!