recommend a jet size Holley 1920 slant six

I'm running a 1945 on my 225 now, but I had a 1920 on it at one time, and it seemed to like a 58 best. It's hard to tell lean or rich by spark plugs anymore (modern gas doesn't color the plugs like it used to). Jets aren't too expensive; I'd buy a 57 and a 58 (they come in pairs, unfortunately) and give them a try.