[Gunner]

At a certain point, carbs that have been heavily modified for special service (marine, high altitude, exacting emissions regs, an unusual engine) are not worth trying to rebuild for other service. You might actually save a great deal of time and money by starting with some stock carbs. 500 ft altitude is within normal service range and should only require minor adaptation to reach perfect jetting etc.

EDIT: A good carb shop might give you decent exchange/core value for your carbs against another good, used, stock pair.