Here's a couple pics of the one I made and used for restoring my '66 Sprite. It worked very well but was only used with the engine and driveline removed. As I started to reassemble the car, It started to get hard to position so before I reinstalled the rear axle, I removed it from the rotisserie and put the car on jack stands.
http://members.cox.net/colin8/Sprite%20Pics/P1020723.JPG
http://members.cox.net/colin8/Sprite%20Pics/P1030080.JPG
http://members.cox.net/colin8/Sprite%20Pics/P1030081.JPG
http://members.cox.net/colin8/Sprite%20Pics/P1030085.JPG
The auto rotisserie is very popular for doing major restoration requiring fitting of new sheet metal to replace rusted panels. I don't think I've ever heard of anyone doing it with the engine installed. Mine used the rear bumper mounts and the rear leaf spring mounts in the rear. I didn't trust the bumper mounts alone even with the car completely stripped. The front attachment is directly to the frame rails and is very sturdy.
If you plan to leave the engine in, the position of the CG is very critical. Mine balanced pretty well fully stripped but it did prefer to sit upright and required a little effort to roll it inverted. Overall it was pretty easy until I started reassembly. Once the interior and front suspension were installed it required more effort to turn. I could tell that once the rear axle was installed, it would no longer be advantageous to have it on the spit..
Good luck and post some pictures - I'd like to see the design of what you acquired.
--Colin--