I agree with Jerry, the distributor sounds 180° out causing the backfire.

The bog however is a different problem, have you considered that you may still have a vacuum leak at the intake manifold? The leak may be on the oil side and not visible from the top. Once you get the distributor corrected you can check for vacuum in the crankcase with pcv disconnected.

Sometimes the surface angles between the head and the intake are out of alignment, check with clay to see if gasket surfaces are even (top to bottom and length wise). If the angles do not match, it will cause all kinds of trouble that a gasket can not fix.