Agree that 30-50 foot pounds is too tight and likely distorted the cover. 20-30 foot pounds would be more like it. Also, since the bolts go into the differential carrier, it is important to put sealant on the bolt threads to keep gear oil from seeping out. One product I like for this application is Permatex "The Right Stuff". I know, a stupid name but this is a type of sealant that works better than routine RTV. You don't get the seepage later on. It has more adhesive qualities than regular RTV so I wouldn't use it where you routinely remove the part for maintenance.

Kent

1937 Chevy 1/2 ton
1942 Chevy 1/2 ton
1947 Diamond T Model 509
1951 Chevy 1/2 ton
1950 Chevy COE Model 5700 ~ "Barney" ~ And more pix