...BTW your Bieber avatar is awesome.-Malone
I would say air in the IP. The cummins pump I bought had loose screws in it and the internal pressure wouldn't build enough to start properly. After starting it would idle poorly and run ok. I would guess that with the old starter it didn't get all the air out of the IP yet. If you could get someone to tow you for about a mile in gear with the key on it will help your starter and that should pull plenty of fuel up for you.
I dont know man, I have had a few of these pumps apart very recently. The squareness of the head is A LOT bigger than the cylindrical piece the o-ring fits around.. I would almost fear you have a high chance of tearing the ring, or deforming it.