71 water pump on 69 block

No issues. the recessed flat face of the tcover is the same for both early/late. Might get the 8 blade (non AC) pump (price should be the same or close) and that'd be an excellent time to weld on an anticav plate. You'd wanna check clearance from the added plate to the tcover recessed flat & want .040 (at least) (040 is perfect/cant have less as it might contact/more reduces vane efficiency), keeping in mind gasket thickness but most setups will have way more than that but it (like everything) always needs checking. this'll vastly improve the pump efficiency. EDIT later pump is thicker so might need longer bolts for the proper engagement & as you know you want proper pulley spacing