Well, I am not so worried about the grade of the bolts as I would be the mounting position of the tranny. It is important that the dowel pins are "intact" and are not floating. If they are you need to replace them as the wear and tear from the aging in conjunction with vibration has changed their tolerance. One of my rear pins was floating in position even though it was still in place it did not function as it was intended. The tranny side has a bit of a cavity for the rear dowel to fit. If it is off or loose, or out of round it will not perform as you need it to.

When I mentioned above about the grade of the bolt, I had drilled out the rear dowel pin hole slightly over bore to fit a 14mm bolt in place of the 12mm bolt and still be able to house the pin. I then tapped the tranny for a 14mm bolt. I was able to use the thread of the rear engine mount (dowel pin hole) to insert the 14mm bolt into place and fastening the tranny thinking that it was going to fix my problem. Well it sheered right through it as well. Not knowing if this was due to torque or not because I just sheered an axle weeks before... I was beside myself.... until... I fixed my vibration problem (cams are mad vibration) the dowel pins, and anything else that causes bolts to snap... or back out.

To make a long story longer... the old tranny is now sitting in my garage... I got a new one...that grinds in second after an hour of straight driving...WTF... but the idea that you are going to fix a problem with going bigger...stronger... fatter... "grade 8" is more than likely not going to fix what you are already over looking... what I mean is... it is not the bolts that are the problem.