Do you perhaps mean Reynolds 753? If so, DO NOT TRY IT!

As one of 3 brazers at Trek qualified to braze Reynolds 753 frames back in the 1980s, I have direct, personal experience with this material. Reynolds states that 753 cannot be cold-set. When a team frame was damaged in a crash, we had an opportunity to check this claim (the frame had to be replaced anyway). We put the frame on our alignment table and used the Levers of Persuasion to try to change the alignment. Exerting force that would ordinarily cold set an ordinary steel frame had no effect; the frame would always spring back into its previous alignment. We increased the force to the point where we had 2 people hanging off the levers. Nothing happened for a while but bouncing up and down on the lever eventually caused the down tube to buckle at the shift lever bosses.

I suspect that spreading the rear spacing would proceed in a similar manner.