That's going to be a BIG job...for the time and money you'll have in parts you might be better off buying a 60Hz set, and keeping your 400Hz set for parts.
Just to give you an idea, I have a 60Hz head from an 003a laying in the garage, but it would take quite a few Ben Franklins to get me to part with it!
There is a LOT more to it and just changing the head. The voltage regulator is different (probably at LEAST $150), the bridge rectifier board is different (maybe another $50 to 75), the CT, CVT is different (someone posted a while ago that the found one used from a dealer for $400). Like I said, till you change all of the various parts, you would be cheaper to buy a 003a and part yours out, or keep it for spare engine parts. Just my