Upgrading stock 1971 318 heads

bigger valves and 302 heads are not gaining a mild cam 318 50 horse, maybe 30. as has been said-measure deck height and do the pistons have valve notches? if the heads have not been recut, cut them .010", or u will need to cut the intake side. if u cut them more than .010" and the pistons are close to the deck and have no valve relief notches-check valve to piston clearance