I have a 540 ci Mega iron block, with aluminum heads. Trying to find out what to set my valve clearance at. I have a 660 lift roller cam. Can anyone help?

I'd start with .026 on both intake and exhaust.

What brand and grind number?
Cranes where around .026 hot, Bullitts are .016 hot and others are all different on the lash like the Isky 660 roller being around .028 if my memory is accurate,
The original Racer Browns (pre 1980s)might have been .032 hot

I have a 612 iron Mega block, B-1 aluminum heads, Crane 648 lift cam, I set both intake and exhaust at .18 cold,
I like a bit tighter lash on all my roller engines, never had any issues, good luck