Hot Rods Structure to replace inside rotted body metal

In building my 39 Poncho coupe into a hot rod,I removed all the interior body metal and floor from bottom to top of fender openings because it was all rotted plus i have a narrowed 2x3 frame and tubs(already in). What should I use to tie together the tail to the rear of door openings? Would you guys recommend 1" square tubing? I want to replace the bracing that the interior body metal provided.

 

1" tube is good stuff in my book. If you have a guillotine and folder its easy enough to make up some C channel for less structural bits, like tack rails. I'd use something a bit more heavy duty for door locks or hinges though.