The crossmember you'll need to find is from a 54 or 55 1st. series truck. It has the angled mount pads that fit your 57 bellhousing. Try putting an ad in the "Parts Wanted" forum to see if someone might have one left over from an automatic transmission conversion. The crossmember was hot-riveted in originally, but a replacement can be installed by using tapered punches for hole alignment and reaming the rivet holes for shouldered bolts. That crossmember also reinforces the rear spring mounts, so simply using regular bolts for installation allows too much frame flex.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!