I think the root of the problem is the knucklehead who used stainless bolts, not you over tensioning the belts. Stainless bolts are only around grade 4 or 5. No wonder they sheared.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked