Bo,
You probably know this oldtimer's trick, but it may help. If you can find a larger valve than you have with the same stem diameter, you can cut some grooves in the outer edges of the larger valves and use them to cut the pockets in the pistons larger. Assemble the motor with the larger valves (no cam required), rotate the motor until it is close but not hitting the valves, then rotate the valves with an electric drill as you lower the valve into the piston. I've never tried it, but it sounds like it should work. Of course, you'll have to deal with the aluminum filings; maybe just grease up the edges of the pistons to capture them. I'm sure someone on here knows more about this procedure.
Tom