Using the actual measured diameter of each wheel, I was able to make a "ruler" in Paintshop so I could get the scale pretty darn close between the pic of the Corvair (which I took some 18 months ago) and the pic of the frame (which I took this morning. )
I also measured the tires themselves, and considering the wear, they're basically 35" in diameter.
So, using the measurements, the above pic is pretty darn close. You can see from the superimposed frame that the body is dropped down just about as far as it can be given the height of the engine. That dimension might be able to be fudged somewhat, since there's decorative covers on top of the intake- and for that matter, I could always add a Chevelle-style "cowl induction" raised portion of the hood to gain another inch or two if absolutely necessary.
However, the actual floor of the car is only about 2" below the bottom edge of the door- the perspective of these photos isn't perfect, but it looks like I might only have an inch or two of room to drop the body before it starts getting too close to the cats and other important bits.
And it looks like the firewall is going to have to come rearward maybe as much as 3", which is not a big problem at all.
Overall, using my "ruler", it looks like I'd need to chop right at 46" out of the middle of the truck frame- and even that assumes I'd be moving the rear wheelwells rearward a touch, to produce a slightly longer-than-current wheelbase. (In that pic both wheelwells and flares have been massaged as well.)
Although the original idea was to end the factory frame right behind the torsion-bar mounts, and fab a new frame from 2x4" box tubing only up to the upper rear quarter of the rear wheelwell, then 4-link and coil-spring the axle.
The only problem there is of course I wouldn't be able to have much of a trailer hitch back there, nor much to bolt a semi-substantial rear bumper to.
So I may keep the truck frame all the way back (I can still 4-link it) or fab that 2x4 box tubing frame all the way back.