Cometic, and mine has to be larger than the bore due to the big intake valves. I think I run a 4.530 on a 4.500 bore, but it might be smaller. Thickness is anywhere from .027 to .041 for me.
Cometic make a head gasket specific to a BMP block. You may want to contact the company that you bought the block from, or Bill Mitchell directly, for the right head gasket. I have a BMP block as well that I haven't put together yet.
I use a .040" thick Cometic MLS gasket. Pretty sure it's 4.53" bore. The KB block has it's own gasket also...may be the same as the BMP, not sure. The steam holes are relocated outward.
the first K-B street Hemi motor I built had to have special head gaskets made due to it being a dry sleeve block, I ended finding out that the race KB water blocks take different head gaskets depending on which version the block is, IE a stage 1 block doesn't use the same head gasket as a stage 4 block does