Will my Reynolds 531 frame withstand spreading the rear dropouts from 110mm to 120mm without a great risk of dimpling the stays? Anyone have experience with this?

I've spread a few 120mm frames out to 130mm, so as long as you have controlled leverage and can bend each side separately while measuring each side's 5mm movement, this is quite do-able.

This frame is a track bike?

Also, to help the stay's resist coming out of alignment while riding, it's best to bend each stay 6mm, then back 1mm. You'll see that it takes quite minimal force to bend back that 1mm, which is merely a stress-relieving process that helps the widened frame hold it's "adjustments" better later under riding stress.

A reynolds frame with 110 as standard might be pretty old, so the chain stays are probably pretty long. Yes, it can be done.

Yes, a 1951 R. O. Harrison track. Thanks for your good tip about stress-relieving.

The frame is a 62 year-old track frame and the chain stays are well over 16 inches.

+1. I've done changes of that amount to Reynolds 531 without issue. The only way you'll damage the stay is by getting too aggressive, usually on the first try. The best approach for a beginniner is to ease into things, gradually increasing your force until you get a permanent deflection.

Oh, T-Mar are you ever right on the first-try-aggressiveness!

10cm? All day long! It's wise to clamp the Bridge so you don't put stress on the joint. I spread this from 110 to 120 without any issue.

I presume that you mean both the chain stay bridge and the brake bridge. What method of clamping did you use? That's a gorgeous bike. BTW, I also have a Maclean Featherweight, 1957, KD403.