i would like to know if heads from a 1974 390 will fit a 1966 390 engine casting # c6me-a on block of 66 engine thanks jim

They will fit just fine.
The '66 and '74 390 heads are virtually identical, including valve size. But the '74 head has a larger, lower compression chamber, resulting in slightly less power.
But you will gain hardened valve seats so it will run very nicely on regular unleaded gas.

BTW, welcome to AF!