I was involved in a pretty serious motorcycle crash end of July, so finally getting well enough to get back to this. I made a prototype crossmember from 1x2 with a .080 wall. The focus was to see how making the cuts and bends would work without having to cut all the way through the tubing. Trial and error, and a really useful youtube video helped make the right cuts
https://www.youtube.com/watch?v=vFdmdC2zxZc. I kept cutting too deep without the offset and had a nasty gap to fill, so this helped a lot. So I ended up with a nice 42" span with a 1/2" drop and TIG welded it up. This way it can tuck up just under the floor and give maximum exhaust clearance. This material is definitely too thin though, when I stand on it it will deflect a 1/4". I'm unsure how much actual weight would be on the trans mount, probably a few hundred? So the next iteration will be 1x3 with a .120 wall. I think that should be plenty stout.