Low oil pressure ;timing mark not stable

Hello I have a 1974 chevy c-10 pickup w/ a350 it has about 40,000 miles on rebuilt motor (upper half). IHAVE TWO PROBLEMS, first my oil pressure is good when I first start truck, but it slowly declines as I drive. When idling in traffic the pressure gauge reads next to nothing, then goes up slightly as I drive. Now I recently noticed a low, not loud thunking coming from the motor, could this be due to my low oil pressure?Is it possible to change the oil pump w/o removing the motor?My other problem is that when I hook up a timing light to the motor the timing mark is not stable at idle. The prior owner installed an HEI of unknown age. Could the distributor need replacing, it also has a vaccum advance, and the motor seems to run fine, just slightly rough at idle.