I had an 80 van with the 250 that I did the same thing you did. If you can get the sheet metal out of the way, or drill a hole in the right place for a long bit do it. I know there are a lot of balancers out there that were hammered on, most of them are luckier than you were. And those vans are rough to work on. As for the bolt size, the new book I got on these engines says to use a 1/2 inch NF bolt. I would get a grade 8. On mine, I made a washer out of 2-1/4" steel bar, cut it about 1/4" thick.
