port & polish ???

Will a port and polish lower the fuel mileage in my 225 slant six?

It wont lower it, but it wont raise it noticeably either. Matching the ports in the head to those in the intake or exhaust manifolds, and polishing the insides of the passageways is going to help an engine that you want to produce more power at very high rpms - like above 4500-5000, and at high throttle opening. Before doing a porting/polishing you would want to have a non-stock camshaft, a modified exhaust system, probably a different carb. (Unless its fuel injected, but I dont know if there are any fuel injected 225's). And possibly a modified distributor or at least a professionally rebult one. And if you are going to ask that much of a slant six - you would want to check the engines bearing clearances. I would think a new vibration damper would also be a good idea.