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.