The shell will not have much torsional or longitudanal rigidity around the door opening with the sills removed. Some ironmongery would definitely be in order before you chop the sills out.
I'd cross brace the shell with 32x32x3 SHS secured by tags to the tapped upper and lower front seat belt bolt holes on the "B" pillar, and bolt the centre of the braces where they cross to give some torsional rigidity to the centre of the car.
I'd also add two 32x32x3, or even 50x50x4 SHS braces across the door opening on the side where you are going to remove the sills. They would be parallel to the sills and pick up the door hinge mounting points on the "A" pillar and the seatbelt bolt holes mentioned above. Take a couple of tags off the lower brace and pick up the outer front seat mounting points to tie the edge of the floor pan in.

The lower beaver isn't particularly structural, but the the usual rotisserie points are the rear bumper bolt holes so the rotisserie might get in the way. Perhaps add a transverse tube between these points on the outside of the car to keep the rear rails rigid and correctly spaced, which would also help when you remove the tubs.
Depending on the scale of the butchery with the tubs, you might want to tie the front hangerpoint of the rear springs back to the transverse tube mentioned above with some 50x50x4 SHS. to keeps things rigid around the rear wheelhouse. You can just poke the tube into the recess where the front of the rear springs normally go and bolt through it.

Good luck.

Check out episode one of Project Binky

https://www.youtube.com/watch?v=7hCPODjJO7s

Now *that's* a rotisserie! In later episodes as they weld in various panels and custom parts they use a spirit level to ensure it's true, all based on the jig being level from day 0, and the shell being completely braced for rigidity. Make the noise!

**** me, that "Project Binky" is funny as.

Thanks for posting.