Shock tower holes
Yep, that was a pretty common 'fix' to get at the upper control arm bushings so the grease monkeys could grease them. It is the ragged, uneven hole that causes the stress raiser in the area that would cause a crack. A nice neat minimally sized perfectly round hole is perfectly acceptable from a structural standpoint. Of course you know the proper fix is the special 90-degree close clearance grease fitting, but who did that waaaaay back then.

Even with the special fitting they are a PITA to get to.
If your shock towers are not rusty -- and it looks like yours aren't, and if you have the engine out for easy access to the towers --and it looks like yours is, it is easier to weld a patch/repair in place rather than replacing the whole tower. If you have access to a MIG welder, and you cut the patch the right shape and size, you should be able to grind the weld down and refinish the area like you would any other weld-in body patch. I don't know about how big the crack is, but that can probably be welded up/ground back down as well.
I have seen some neat looking holes cut into the towers in this area. Some even look die cut they're so perfect. Then all they have to do is fit a rubber plug in there and it looks factory and still gives easy access to the grease fitting.
The choice is yours!