Here is a picture showing the (2) 5/16 X 24 grade 5 bolts securing the bottom of the timing cove on my 228 GMC.
timing cover bolts Notice the copper washers to prevent oil leaking through the bottom of the cover. I suspect the GMC 228 is similar to the Chevy 235 in this area. Also notice the tabs that are bent over the bolt heads to prevent them from coming loose. The remaining screws that secure the cover are combination 7/16" /Phillips head screws that are attached from the outside of the timing cover. I would also recommend using Permatex #2 Aviation gasket sealer on all your gaskets to help prevent oil leaks between the engine block/front motor mount/timing cover.
