OK, well I already welded the trunk floor in, and I'm still second guessing myself about the frame rails. I could not get the welding tip into the rails to weld those plates completely around, I tried.
The plug welds into the plates inside the rails were 3/8 inch, they seem really strong. Also, I am going to weld in either 14 gauge or 1/8 steel, bent at an angle, from the rear of the trunk (rear cross piece) to the tail panel since that section did not come with the two piece trunk. Trunk floor is also plug welded every two inches along rails.
I need to keep moving on this project and the rails held me up for awhile now... but if I need to cut them out, it needs to happen now before I get the drop-offs welded on.
Thanks for your replies.