I started this thread in the fixed gear section, but the bikes are all vintage so I'm putting a link here if anyone from the vintage section has any input.

Which frame to fix?

Think about a Dingle speed
a 17-21 2 speed freewheel and a 52-48 chain rings.
IMG_20161021_144452736 by Bwilli88, on Flickr

you could get a Surly Dingle fixed gear cog in 17-19, 17-20, or 17-21.
I would like to change my dingle speed into a dual dingle speed with the 17-21 freewheel and a Surly Dingle cog on a Fixed/Free flipflop hub, now just to find a dingle cog in 17-21.

Saw your thread over in the SS/Fixed subforum.

I'd opt for the Trek 560 or Miyata 710 to try your hand at a fixed conversion. You mentioned the 710 feel somewhat stiff...is it larger than the other frames? We all have our own preferences, but when riding fixed I prefer a frame that pushes back when you lay down power, not one that gets noodley. Sounds like the Trek or Miyata could both serve you in that way.

The Cannondale would probably a bit lighter, but would probably be a more harsh ride than the others.

This is PERFECT. I'm discovering as I rev up to this project I'm more intersted in the "fixed" idea than in the single gear idea. I'd really want at least two, one for railing around and one for noodling around.

I would like to rebuild this concept on another bike. I will use possibly a 73 Raleigh Super Course. Right now it is a 76 Raleigh Gran Prix.

The rear chain stays and dropouts on the Trek and Miyata are too short to get a dingle speed to work nicely. For this you would need a frame set with fairly long slanted dropouts and bit more chain stay length to allow for changing between gear setups without changing the chain length or switching chains.

Also the longer the stays the better your chainline will be. Personally I am a purist and the desire for an optimal chainline would trump the desire for two cogs. But I also live in sort of ideal fg terrain.