Pretty sure the oil pump gets 18 ft. lbs. of torque on the 4 bolts. You SHOULD NOT stick a larger bolt in there, if the holes are stripped you'll have to split the case and basically rebuild the motor, and maybe save the case with time-certs.
Usually, oil pumps have studs in there and sealing nuts on the cover. And you MUST use a thin paper gasket, if the gasket is too thick you'll lose oil pressure.
Buy a 66-69 VW Beetle service manual by Bentley, it will serve you well.
You have a universal/type3 case. The two-bolt flange is not a big deal to seal, use whatever method makes it seal.