Are you absolutely sure that the engine is HO? If it is it will have the 351 firing order and those flattops are forged. I have seen where someone was running an E303 in that block and he did leave some marks on the pistons. A few more rpms and it may have detonated. A lot of the HO engines out there are really just the 86 and newer roller 302 for passenger cars. You can make it an HO with pistons and cam though. The 96 Explorer came with GT40 heads and the 97 up came with the GT40P heads on the 5.0's. You can make a lot of hp with E7TE heads with port and polish work and a thinner head gasket to bump the compression a tad. You can do it with about a .500" lift too, not necessary to go much more for the street.