You could find a tired-looking (but structurally sound) '70s P15-9 Paramount frameset and have canti / center pull posts brazed onto it for 700C or 650B wheels and go from there. P15s have a 44.0cm chainstay length (measured to the center of the dropouts). If any P15 is like mine, the frame feels very strong, but provides an excellent ride with confidence at any speed. They'll take 700x35 tires (on a 23mm wide rim) and likely wider with a 650 setup. As I've stated a million times around BF, mine was tired, received repairs, and is my best bike.
PX-10s of this era are pretty much the same in spec, if my briefly-owned '74 64cm version was anything to go by (73° angles, 44cm chainstay, DB 531, Nervex Pro lugs, 60cm TT).
Cimmarons may be well made (they get their name on the headbadge, so that's a good thing), but having ridden and lifted one, I am not a fan. Depending on the riding you want to do or like, you'll have to see. I found them incredibly heavy. Their ride and handling can be characterized as "lumbering." Now, I would imagine that a newer, stiffer set of 26" wheels and some killer Compass tires would shave tons of weight of rotating mass and greatly improve the responsiveness of the bike. From there, you still have to deal with geometry, so maybe it still is a '70s Cadillac in the movement department. Vintage MTBs have some odd proportions, especially at my height. A touring bike always looks better for big tire duty.
