I bought one of the Vevor 66-pound London pattern anvils based on favorable reviews of budget anvils. It seems pretty lively using the ball bearing test and is reasonably well finished-- except for the hardy hole. Not only is it out of square but it just barely measures 3/4 inch at the widest. Is the traditional fix just to take the time to file it square, or does one drop down to a 5/8 hardy tool -- or file the foot of a 3/4-inch tool to fit the hole?

IMO you are better off to file and true up the hole to 3/4". There probably isn't much you can do with power tools unless you try a Dremel or somehow rig a file to a reciprocating saw. I'd be VERY careful about trying to use a metal cutting blade in a reciprocating saw. Just resign yourself and get into the zen of vertical filing. It might go a little faster if you turn the anvil on its side so that you can file horizontally.

"By hammer and hand all arts do stand."

You can also get a "bandfile" or "detailing file sander," which is a very narrow belt sander and should fit in the hardy hole. That might be your only choice if the anvil's too hard to cut with a file. There's one for less than $30 at Home Depot, though I hesitate to think about what you'd end spending on abrasives.

If you are able to file, I'd suggest a 1/2" (or better 5/8") square file. You'll need quite a bit of pressure to cut steel that thick, and a square file would give you the stiffness you need.

I don't think I have any store bought hardy tools, so if it were me, I'd just make tools to fit the hole.