I have ran into this many times.

Thats why you replace the bolts with compatible 13mm hex head bolts for the next guy when the water pump seal goes bad again.

I use a spline drive socket on a 1/2 inch drive long(wobble type) extension (not universal )type and pound that sucker in there with a Big hammer. (Power steering pump unbolted and moved back away, lines still attached.)
I also apply a little bit of valve grinding compound on the end of the spline drive socket, that keeps it gripping and not slipping.

It works every time, just make sure you use the right size spline that will fit into the hole snug,and don't be afraid to whack that sucker in tight with a hammer.

Remember, auto parts stores let you borrow tools.

make sure you replace all the bolts with the right length metric thread bolts with a 13mm hex head and use wavy type lock washers.

That's how the old man taught me how to do it.- Works every time.