will 496 exhaust bolt to 500 hp heads or is a diffrent bolt pattern.

Just for kicks I bolted up my 496 HO CMIs to my new 525 EFI. The ports lined up perfect and so did 4 of the bolt holes. You would need to re-drill 4 holes per side and they should work. This is with headers with a flange. Of course cast manifolds do not have a flange and cannot be redrilled.

The 525 is different and has special heads and different exhaust bolt pattern.

Merc 496 exhaust will bolt right up to 454/502 rect port heads.