Kewl setup, Gypsy. I put a Heidt's set up on a deuce frame, works great!! Kind of what I figured you had in mind. Go to the Wescott site, they have the measurement for the '34 frames on there. Set your jig up to the stock specs with it very secure to the table from the firewall aft. I would place fixtures on the table from the firewall forward, about 12" apart and placed on the outside of the rails. Remove the front x-member and began squeezing the rails equal amounts on both rails by shimming and block between the fixture and the rail. When you get the amount of squeeze you need for the new front x-member, tack the x-member in place and check your frame measurements. Be sure to move both sides in equal amounts, work slowly, measure twice, cut once.

Vara, I made my own corner clamps (I assume your are talking about rectangular or square tubing) using 2" X 2" square stock and the appropriate sized C clamp. I fasten these to the table top by either a bolt from the bottom or a tack weld. I hate it when things move around on my jigs !!!!