LS5 454 engine questions

I just got this new 1966 chevelle. It has a 1971 LS5 454 block according to the casting number on the block. The horsepower of this engine is supposed to be 365 HP. I am having it rebuilt now and when the head numbers were run it turns out that one head is off a 1972 engine and the other is off a 1975 engine. Will reusing these heads make the engine any different regarding horsepower? I am having it bored out and will be adding a different cam just enough to give it a better sound. Everything else will be pretty much stock. It does have an Edelbrock Performer RPM intake and a 750 Edelbrock carb. These different heads have me worried.
I am new to the world of high performance engines. Any advise on what I have?

Hey man, I strongly recommend getting new matched heads man like for a '71 ls5 because, those heads are rated differently as far as horsepower is conserned. In 72, the horses were around 300, in 75, try about 225 or 250 tops. So man i'd get me a set of aluminum heads with big valves since you're boring it, it will work fine. You do that you'll be crankin about 4 + man.

After researching the numbers on the heads today I descovered that these are the correct heads for the LS5 engine according the the casting numbers. The numbers on both heads are 3993820. They are oval port heads. I also found some other numbers on the heads that I do not know what they mean. On one head it says GM9T and on the other it says GM11T. Maybe it just meand one is a right and one is a left. Not sure. Anyone know what these numbers might mean?

The casting numbers are the same, the heads started life the same. Be certain to CC them to make sure one side hasn't been milled more than the other. Differences in chamber size can make an engine run terribly.

What does CC mean? I found out that the motor has allready been bored .060 over but the machine shop thinks it can just be honed because it looks to be in good shape. Is there a disadvantage of the moror being already bored .060 oversize if it is useable?

Just because the casting numbers are the same doesn't mean the heads are the same. That particular casting number can come with either an open chamber or a closed chamber. Basicly, in the combustion chamber the area around the spark plug hole will be built up or "open". CC stands for cubic centimeters.