There's lots of vendors that will custom build a frame...TCI totalcostinvolved.com for one. fatmanfab.com is another. Best to pick up an issue of "Street Rodder" magazine that has lots of vendors advertising their produce.
BTW I've own a '31 Chevy 2 door sedan since '68 and hopefully will "resurrect" it in the near future. Here's a pic of it back in '80.

PIC