I picked up a Reynolds 520 frame at a good price, but I wanted to reduce the reach by 1.5-2” Is this possible or will it ruin the geometry of the bike? Is it a relatively easy job?

It is a near impossible job, and even if you could it would be an incredible redesign of the frame.

If the stem is 100mm just put a 50 mm stem on it. Or if 120 mm stem a 70mm stem. That will reduce reach by 2" inches (50.8 mm).

Otherwise your question seems more appropriate for the Framebuilders sub-forum if you are wanting to take apart the seat tube, down tube and top tube of the frame and re-braze most every lug and tube on it.

It can be done, but if it doesn't cost more than what you paid for the frame, you paid too much for it.