Your assumption that the rear crossmember is important for supporting the spring mounts is spot on. I responded to your parts wanted post in the Swap Meet forum and forgot about that aspect when I recommended to just buy a tube style crossmember with V8 motor mount bosses on them. You will also need a crossmember at the location of you are currently missing the riveted on crosmember.
My revised recommendation is to forget about the tubular crossmember for mounting the engine. Go online to chassisengineeringinc.com and order part# CP-1160. I have used this setup on a '49 and a '50 with 350s and it was painless and it places the engine exactly where it belongs. It is not a crossmember, so you will need a tubular crossmember installed near where the original crossmember belongs. I recommend a bolt-on tube crossmember because you will need to be able to take it on and off should you ever need to remove your transmission. This will also make servicing the transmission filter possible.
You could just bolt on an original crossmember by drilling out all of the rivet holes, but you will find that when you level the V8 engine on the frame, you will only have about 1/8" of clearance between that crossmember and the transmission. The tube crossmember won't cause this tight clearance issue.
I apologize for the incomplete/totally wrong recommendations given to you in the Parts Wanted Forum.
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission