318 oil pump replacement

The oil pump went out on me yesterday.i have a 74 duster with a 904 auto i just had the engine out about a month or two ago to do a freshen up and tune up on it and i decided to go on a madin voyage yesterday to the gas station and i noticed it went from 25 psi to 0... What can cause an oil pump to go out? I should have changed it out when i had the motor out but i figured it would be fine. It was working ok when i pulled the motor than it started to only give me about 25 to 30 pounds at about 30k rpm's and now nothing?????wtf...and it looks like its going to be a pain in the *** too pull the oil pan with these damn hooker long tube headers in the way not to mention the tranny cooler lines are in the way too aggghhh is there an easy way to get the pan out without pulling half the engine apart?