Howdy Back:
The '77 head will have bigger intake valves at 1.75" compare to 1.69" on '66. It will also have a larger volume intake tract and a 1.75" carb hold as compared to 1.5" on the '66.
The chamber volume on the '77 head will be aproximately 62 ccs in OEM condition (Before any milling). a '66 head comes from the factory with about 52 cc chambers.
The '77 head will have hardened valve seats. If you were doing a valve job on your '66 head to add hardened seats it would cost at least an additional $50. IF you need to do a valve job stepping up to the later head would likely save you some money.
Either head would benefit equally from a three angle valve job. A three angle valve job would generate the a bigger flow increase then the increase in valve size.
Both would need a .025" mill cut to compensate for the thicker aftermarket head gasket. The '77 head would need an additional .050" to return CR to about the same as a stock '66.
If you can utilize the larger carb hole with an increased flow 1101 or RBS carb, and increased intake volume you will see a performance increase. The hardened valve seats offer a long term durability plus.
To repeat what Wallaka said, from the C9 (1969) head casting on 200 and 250 heads are identical per casting code. A head with a D7BE (1977) casting code will, and has been found on factory original 200s and 250s.
So, the answer to your question, "Would me swapping the heads do anything to the performance of the car?" The simple answer is no. Without compensating for the chamber differences you would actually lose some performance due to less CR, and larger intake valves cannot make up for that loss. However, the potential to perform is greater with the later head. With milling to regain CR, a larger carb to take advantage of the larger intake, a three angle valve job with back cutting the intakes, The later head has an advantage.
As always, it's never just one thing.
Hope that helps you.
Adios, David
