I think that thru-bolting the lower bolts would be fine. Be sure there's clearance to the clutch pressure plate. Just a random thought: Could the holes be tapped for a helicoil insert? Not sure if there's enough meat for the insert though, and a helicoil kit of that size would be a bit spendy, although knock-off kits are pretty cheap. Link


Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.