I have a set of thru bolt Harman for 350 with snails. The snails still have the step down an you can cut the lower off to gain 3 1/2 if desired. The thru bolt will allow for a better seal an give you the ability to use a good felpro gasket. The thru bolt also has the water jacket going up to ports which allow for a more controlled log under heat as ford heads have weak cooling around ports. This is one of the reasons the flange type log will warp an leak at ports guaranteed.