When this kind of thing happens, an angle drill has saved me many times.

I have also drilled clearance holes though fenderwells to get a very long drill bit straight onto broken bolts.

You can try running it without this bolt, the most that will happen is you'll get to take it apart again and drill that bolt out.

 

I did the same as 1994 Sport. I bought a right angle DeWalt drill at Home Depot and some extra centerpoint bits. It was well worth the effort to me. When I drilled it, I was not quite perfect, so after I got it drilled, I tapped new threads and then drilled the new water pump to match before I installed it for good. It is still holding after many thousands of miles.

If you choose to NOT install that bolt, you are leaving essentially half of the face unsupported and I will almost guarantee you that it will leak.

That is the ONE bolt that has the back side of the threads open to all the road grime coming from the front.

 

I liberally applied rtv sealant to all surfaces and to o ring. Installed, let it cure for a day. It hasn't leaaked any water. I check it cold by removing radiator cap, and it is completely full. Been that way over a month.

 

Same thing happened to me a year ago. I bolted the new pump in with the remaining bolts, didn't use any RTV. Its been a year, no leaks. You are good to go.