you do know that the distributor gear must mesh with the helical gear on the camshaft, right? This means that, in the last inch of travel when you insert the distributor, you might get a bit of resistence as the gears mesh..... and also, the rotor will turn a bit as you slide it on.
If it really is the oil pump shaft, look down the dist shaft bore into the engine. The oil pump shaft should be centered in the middle of the hole, as you look down. If it is leaning to one side, jack up the car a bit and move the shaft until its centered.
If the shaft is centered, just rotate it a bit with a screwdriver and try again, until you get it. The Chevy 'slot' type drive is much more difficult to center than the Ford style of flat-sided pump drives.