There was a problem with 753 chain stays breaking when it 1st came out. I had 2 Raleigh frames come through my frame shop that had cracked chain stays. That scared me off of getting my 753 certification and I still have a check for £75 in my desk made out to Reynolds to pay for a 753 test tube kit. I never sent it in after seeing those cracked stays. A frame I made with cracked tubes could be very expensive in money and time to fix (bicycle tear down and rebuild, replace chain stays and repaint) and my reputation could take a huge hit even though it wouldn't have been my fault (and then the customer might still insist on a new frame anyway). At 1st Reynolds required you send them a complete frame for testing. When I talked to Terry Bill in Birmingham at their factory in 77 or 78 (I don't remember exactly), he said every American up to that point had failed their test. Jim Merz was the 1st and Rich Gangl the 2nd American builders to pass the test. Right about then, Tange came out with their Prestige line that mimicked 753 so I used that instead for my light frame builds. Later when Reynolds revised the test to be easier, I applied and got my 753 certification. I understood by then they had fixed the chain stay problem with added material and probably less treatment.
Because your frame is made for 27 1/4" rims, I would be suspicious it was early in the era when 753 frames were made and therefore more likely to be before they altered 753 chain stays to keep them from breaking. With my history of 753, I certainly wouldn't take the chance of trying to indent the chain stays. There isn't that much difference between a 30mm tire and a 32. You don't notice the difference as much in back as the front anyway. Besides we are frame builders that can make modern wide tires fit the frames we build.
My understanding of Columbus Aelle tubing was that it was not heat treated like SL or SP. I don't know if it was a different alloy. This you can see in the surface of both kinds of tubes. The SP/SL tubing has a bluish/golden hue while Aelle is plain gray. I remember looking at a video of Columbus SL tubing being made at the Columbus booth at the bike show in New York in the 70's and they showed a black screen with the words "secret progress" - which I assumed later was their heat treatment process. My point is that Aelle tubing would probably be more ductile and more likely crimp with success.