I've got a Unisaw that I want to attach an extension table to the rear of.
There's two holes (approx 3/8" diameter) already drilled in the rear flange
of the cast iron top (it came from the factory that way), but they're not
tapped; just through-drilled. They're a little too small for clearance of
a 3/8" bolt; it would be nice if they were whatever the tap drill diameter
for a 3/8 UNC is supposed to be, but I haven't measured them exactly, so
I'm not sure.

I suspect the easiest way to attach something would be to tap those two
holes and bolt on a piece aluminum angle, to which my plywood table would
get attached. How difficult will it be to tap these holes? I've tapped
holes before, but never in cast iron. Does it go easy? Is there any risk
that I'll damage the top? What if I end up having to drill some additional
holes in the flange?