Basic sophomore engineering here. If you have to distort a steel beam until it bends, 1) start with a skinny one. 2) stay away from very high strength steels with little margin between yield and breaking.

OP has a Raleigh Competition. 531 steel. Highish strength but nothing radical at all. Now his chainstays may be regular Reynolds chainstays for diameter but they may also be the butted chainstays the Competitions were known for (see Gugie's photo above). (Raleigh built those bikes with both.) Those butted chainstays are barely bigger than pencils. Between that and the 531 steel, I am quite confident you could stretch a 120 OLD to 140. 130? The bike's not even going to blink.

One caution - this Competition could be a bike boom Raleigh. Anything in the wide spectrum of quality control is possible. Mine came with so little braze in the lugs that the alignment was perfect. No heat distortion. A local framebuilder filled all the lugs like he was building a new bike. They drank the braze like they were dying of thirst.