bought a mystery 850 double pumper...

if it ran in Colorado, you can bet the air bleeds were altered. First thing I would do is get it all back to stock Holley. Then start fiddlin'. Butterfy size should be square 1 3/4. 1-9/16 bore on a 1 3/4 base is a true 850 across the board. Holley (R80509) 830 (NASCAR mandated 1-11/16 butterfly class) uses 1-9/16 (750-950) venturi and 1-11/16 (750) throttle bores so you can write that off if your butterfly is a 1 3/4. .
http://documents.holley.com/techlibrary_carb_numerical_listing.pdf