The cars I worked on that had solid mount transmissions were all pro stock chassis (Don Ness). My current chassis is an Alston Eliminator I upgraded to Eliminator II specs. It is certified to 7.50. I suspect that it's as rigid (or more) than the Ness chassis cars I used to deal with, but I'm still going to use a polyurethane mount for the P/G. It's no trouble, I have the mount, and the only adjustment I might have to make is to raise the motorplate a half an inch to keep the geometry correct. No big deal.
I appreciate the comments. Thanks to all.