Thanks PhyttaM, Xusia, Nubie, and RM1SepEx for your time to reply and comments!

AZPete replied to the question about the distance from frame to the center of the side driveshafts in my build thread, his pic is from a 2006 5MT in 818S;
image.jpg

This is almost exactly what I have with the trans pan sitting directly on the frame. So I guess I'm going to leave the frame under the trans pan intact. When the trans mount bushings arrive (for a BMW) I will build the height to be about 1/4" space, then raise as needed to keep the pan from contacting the frame under load. I also plan on adding adding a brace to the top of the engine to keep it stable.