Question about 383 pistons

[66Satellite47]

Without replacing pistons, close chambered heads is the way to go. You have a 66, do you know if those are the stock heads with the car or what # they are? They could already be the 516's! If they are, your better off drilling out for the larger exhaust valve and do a good port job on them and get them to breath. By the time your done if you don't DIY it, the coin you spend on those you could get these and be done! Good Luck

http://store.440source.com/Stealth-Aluminum-Cylinder-Head-COMPLETE-SINGLE-HEAD/productinfo/200-1055/

OP says they are rebuilt 906's. Going to the 516 with pocket porting & bigger valves (2.08/1.74 or 2.14/1.81) is a major boost over a stock rebuilt 906. mostly from the porting, not CR increase. Pocket porting the 906's with significant milling to raise CR. But I do my own porting which makes it cost effective. Modern heads are sure less costly than paying someone to improve old iron! Old iron works if you DIY.