NEED HELP,FUEL PUMP BOLT ISSUE,LEAKS OIL

Hi,I’m having another problem now with my 1968 Dart. It has a 360 in it. Anyway when I bought it I started noticing a leak under it. I assumed it was gas but it’s oil. I replaced the fuel pump today with a new one and new gasket. When I went to remove the rear bolt,the one closest to the firewall,I noticed it wasn’t tight like the one closest to the front of the car. I made a note of which bolt came out of where. The longer bolt came out of the side facing the rear. I put in the new pump and gasket and went to tighten the bolts. The front tightened down fine. The rear is stripped and just spins. That’s why the car was leaking oil (which I assumed was gas). Any ideas what I can do?
What size threads are the original bolts? Should I go a size up? How long is the back bolt supposed to be.
So far it’s been one thing after the other with this car. Don’t know how someone managed to strip the bolt. I just need to get this fixed so I can drive it.
Seems like you are new to the car, why not get what you need to replace the timing chain and water pump and bond with your car. You will learn a lot and you can fix your fuel pump problem while you're at it.