Stuart has it right. In fact having the two bottom bolt holes drilled and tapped is a godsend as it saves you pulling the pan to access the bottom two bolts. Pulling the pan, especially if the gasket is good and holding might only create more problems.
On these engines there is also a oil slinger which mounts on the crankshaft inside the timing cover. This "slings" oil away from the seal where the crankshaft pulley comes through.
When I did mine I replaced all the slot-head screws which hold the cover in place with small hex-head bolts. Makes it easier to get to them and there is no slipping of the screwdriver.
Bill


1949 Chevy 1/2-Ton
"Sedgewick"
In the Gallery
1989 Caprice