When you get to cam installation bear in mind that on Chevrolet 216s and 235s if the cam stops turning the crank will turn about a third to half a turn before a counterweight on the crank hits a cam lobe. I don't know if 248 GMCs are like that but I'd sure check before I used either the starter or a bar to turn it over if there's any doubt about cam timing.
On drilling the main cap to screw the timing cover bolts in from the outside, it's a handy shortcut, and a previous owner did this to mine, but that means that you have to tighten all the bolts before you put the balancer on so there's no way to center the cover and seal on the crank. The factory way you can put the balancer on then tighten the two bottom bolts from the inside insuring that it's all centered. Maybe this shortcut was done because fiber gears wear out quickly. When they strip you can kiss the cam goodbye. I had this happen to me far from home. It was stressful.
