What a disappointing night. My timingcover/waterpump horror. Please help!

I can't beleive this crap. A month ago my waterpump started leaking so I went to replace it and ended up breaking the long bolt that goes through the timing cover right underneath the tensioner. So after a month of debating if I should just pull the engine and go to town on it I decided I'd do the smart/economical thing and just take the timing cover off the car and fix it. So after the past few days I put the new timing chain on it and got it ready to put all back together tonight.

First of all, the ARP timing cover / waterpump bolt set I bought for a 302 has bolts that are way too long. Unless they expect you to use 5 washers on each bolt . Thankgod I didn't buy the stainless set for $40.

Now the good part. When I put the very bottom bolt back on the water pump I demolished like every thread in the timing cover. I tried using a slightly shorter bolt just to grab a few threads but had no luck either. Everything just falls right out. The good part is that it doesn't leak, YET. But having that bottom bolt not there just doesn't seem too comforting. I don't want to be driving down the road next month and spring a leak. So what do I do The only thing I can possibly think of is to try and clean the threads with a 5/16" tap and try to get the bolt to grab again. If not, do you think it's possible to tap it with a 8mm tap which is a hair bigger and use a metric bolt? I think thats the only possible option because I can't see a 3/8" bolt fitting in there. I figured if I can't get it with the 5/16 then it's screwed anyway... I really don't feel like ripping it all back down again after that, it would majorily suck.

Usually, I wouldn't be so stressed out about getting it done but my other car is leaking trans fluid like crazy, and the one I'm driving now doesn't feel like it's going to be driving much longer. I was kind of hoping to get my car done fast.