If you have to tighten bolts into the holes, I would go with the slower-setting stuff, as it has nearly twice the strength of the 4-minute version. Neither is going to be very strong. Any threads you manage to get in is will strip if they become load-bearing. From the looks of the pictures you posted, you'll probably still have enough thread engagement in the casting to hold the rear cover on, but that's about it. If you mix it and let it get the the consistency of a putty, you could probably remove the bolts after you get it in place and let it cure without the bolts in the holes.
And this is one of those things where if it were my tractor, I wouldn't even be screwing with this...it would be getting a replacement transaxle.
I'm surprised they haven't had claims filed against them for false advertising...an epoxy with 10% of steel's ultimate strength is not "as hard and tough as steel".
