As for cars that would fit a 5.50-5.99 pound class:
The very rare triple-turbocharged, six-cam 231 EFI Oldsmobile Jam-Pak motor. I tried to run it in the Hemi Challenge at the 2006 Dutch Classic at Maple Grove. They wouldn't allow me to do so, but as you can see here, I showed THEM.
And, a 7.0 and 7.5 pound SS/GT class would be neat, for those wanting to run hi-powered engines (LS-6, ZL-1, Ford T-bolt motor, Hemi, etc) without weighing 4,000 pounds.