[sofield]

I will make the assumption that you are using gland rings in the four exhaust ports and the rings won’t allow the manifold to seat properly between the manifold and block causing the gasket to leak and blow out.

Remove the manifold and clean both block and manifold surfaces. Check both for depressions in the ports. Clean them out of any carbon particles, reinstall the manifold with the gland rings in place but without a gasket and snug the manifold until you feel resistance then stop. Take a feeler gauge and check the clearance between the manifold and block. If there is no clearance use the gasket of your choice. I prefer the copper ones. If you have a clearance use a gasket that is a least X2 what the feeler gauge reading is. My reason I do it this way is you don’t know to what extent both surfaces have been milled in the past. PS: disconnect the muffler first.
Sofield