Need a bit more info. This assumes that you have set your base timing to factory specs.
What did you have done with the heads?
Did they deck the block?
Do you know what your compression ratio is?
Do you have a temp gauge and where is it running?
Where is your base ignition timing set?
If the heads were milled and/or the block was decked, then it raised the compression. If you're above 9.5:1 you'll need to run mid grade or premium gasoline.
You might want to check the amount of mechanical advance in the distributor. If it advances too fast too far, it can give you problems.
