He is not using stock 56 front mount is he? He has a 72 350.
Let us know if you have long water pump. 6 15/16 from mounting surface to hub. That is a key to bracket type.
1972 is right on the transition for water pumps. If you have short water pump, you may need to get the brackets shown in coilover pic. They use the exhaust manifold bolts.
If you want it on other side, it is possible but pretty expensive. Do you plan on making a bracket or two or buying? The stock 1972 350 used what is called high mount alt brackets as coilover pic. In 73 I think they went long water pump and moved alt to pass side and tucked in down in closer to drive pulley.
In the 60's is when GM started using alt instead of generator....so at first they just mounted them more or less like gens.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn