Some ideas for a Fixture....

While (mindlessly) DA'ing the multi-coats of paint off the other Cutlass fender I got to thinking about another of my projects, the old Buick....

Ok, it's a '54 (I think) Buick 2 door hardtop that I plan on turning into my version of Harley Earl's "Wildcat" cars he designed in the early 50's. Ultimately the car will be a 2 seater with a removable hardtop powered by a late model El Dorado engine and drivetrain (front wheel drive). To accomplish this I of course will have to shorten the body considerably, about 18" in all. I plan on doing this in 2 places, the first being just behind the rear door jamb where I plan on taking out 12", then again just behind the rear wheel well where I'll cut out the other 6".

So, here's where the planning comes in........
The body of course will be very well braced prior to any cuts being made, side to side, top to bottom, and a couple of X-braces. I want to do all the cutting while the car is on the rotisserie so I'll need to build a fixture to hold the body and attach to the rotisserie, then after removing the floor and making the first cut, I want a mechanism to allow me to slide the back of the car forward and tack things back together, make the 2nd cut, slide the rear section ahead and tack it together, then continue welding the 3 sections together while on the rotisserie so I can rotate the entire body and do all the welding.
I don't need anything real elaborate (aka expensive)to slide the sections forward, but I would like something more solid then just the usual tube in a tube setup. Some sort of slide mechanism that keeps everything in place but is also easy to operate and not in the way of all the cutting, welding, and grinding the mods will require.
I'll also have to shorten the roof and make it removable, but I'll save that for another thread. Any good ideas on what type of mechanism I should build??