Thermostat and housing

What year t-stats are you asking for at the parts store? In the late '70s Mopar went to a small diameter (Chevy size) t-stat on the smallblocks, and still used them later on the Magnums. Those won't work on your manifold!! You need the earlier size (I showed the listing for a 1970, for example).
Show us a pic and tell us where it's leaking- gasket surface or bolt holes?