should be the same as a big block Chevy... and small block Ford... and even some Chrysler products... since Carter made all the fuel pumps for everyone, they probably told them to make all their mounting flanges the same to make their job easier..
if in doubt, go to Rockauto or Summit or whatever your favorite place to look up auto parts is and look up fuel pump mounting gaskets for the 231 Buick and then look up other applications to see which ones use the same gasket... if they use the same gasket, then they use the same blockoff plate...
or just get some 1.8" steel and make your own with a drill and grinder..