Can anyone tell me which GM frame if any will fit under a 56 Special? We want to be able to have disc brakes etc. Plus if I use something newer parts would be easy to get.
Any help will be much appreciated.

there is not one that is gonna "fit". Your best bet is a GM 'A' body chassis. We have used these under 50-51 mercurys, and the 56 Buick body would be easy enough to modify to fit that. With my 57 Special, I replaced the front and rear suspension with 80 Jaguar units. Much easier than replacing the entire frame.