jw bell or case for sure. same as above with checking for a chassis twist problem. also be checking that driveshaft. remember even if it looks straight it could be twisting out of round under power if its too thin wall. real common problem with aluminum driveshafts. Had a guy with a 63 vette tube chassis mcamis car with aluminum mark williams driveshaft, 632 bbc big chief heads, twin toilets. he broke two cases before he told us about his aluminum driveshaft. installed a chrome moly driveshaft and never had a problem again.