So I checked. Wheelbase for the 250 GT and the 1969 Beetle are both 94.5 inches.
With the long trunk overhang you could easily fit a Suby 6 in the back. It'd probably be best to build some kind of huge "Jack-in-the-Box" thing with a styrofoam V12 to put under the front hood (after the car is otherwise fully sorted, of course)...
I think it's a perfect project for someone like @WOLFGANG