"the 625 mountian gun looks alot like a model 66, are they the same size K-frame?"
No, the M-625's are N frames, both ACP and Colt. S&W doesn't make, and never has, anything double action in .45 caliber that was on anything but an N frame. I don't think, even with a 5-shot L frame (L's are sized between K's and N's), there is room for a .45 caliber in any S&W but an N frame.
The Mountain Guns have a slender, tapered barrel with just an ejector rod shround under the barrel that looks kinda sorta like a Model 66, while most of the other M-625's have a full-length barrel underlug, like a Colt Python. The Mountain Guns thus weigh less, are handier, tote better but kick slightly more; I don't think any factory .45 ACP or Colt ammo kicks hard enough for the extra weight to matter, though.
S&W has made .45 ACP revolvers since 1917. Lots of great choices, 1917, Brazilian, 1950 Military/M-22, 1950 Target/M-26, 1955 Target/M-25, M-625 both underlug barrel and Mountain Gun...