I agree with all of the above, I use SS bolts on all the water pumps on our Flatheads. Like 37 Coupe says, make sure you use anti-seize on the SS bolts.
Make sure the bolts are not to long. I have seen a few that people used longer bolts that bottomed out and the gasket leaks. Also make sure the lock washer don't drop off the internal bolt and wind up stuck in the impeller. One of Skips customers had this happen last week. G.M.
SS with anti seize on threads is the way to go for that one internal water pump bolt. It can be a real pita removing a plain steel bolt that has broken off inside the block. Also dealing with a plain steel bolt that has most of its hex head rusted away can also make you say some very bad words