I have run powerglides with a mid plate and no cross member mount at all for over 15 years and never broken anything . however my driveshft is 6" long , if I was using a long driveshaft as in a door car , I would run a mid plate and a GM rubber mount on a crossmember ..just my two cents , for what it's worth (probally 2cents) :D

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.