x2 on the 1/4" cork gaskets for the "U" seal on the pan. ive never used the 1 piece pan gasket but if it has rubber for the U seal its a crap shoot of sealing up, especially for the older 3 slot pans.
i have an aftermarket pan that uses the older 3 slot type seal, i had my machine shop assemble the short block since they offer such a good price & some type of warranty, but i do the rest of the assembly myself. after a fresh build of a stroker motor, i had a minor leak at the rear of the pan like yours, thought it was the rear main, the shop had me bring it back & replaced the rear main then reinstalled the pan using the same felrpo gasket i used, it leaked worse than the 1st time!
so i pulled the motor again & brung it back, they inspected the main seal & said it looked fine, so left it alone but this time i brought them a 1/4" cork gasket for the U channel in the pan after talking with mr Pbody & reading about it on here, they reinstalled the pan with the cork gasket & used ... ULTRA BLACK RTV... as was suggested. the pan has been drip free for about 7 years now with lots of street and drag strip use. if you pull the pan, use the cork gasket for the "U" explained here.
https://pontiacstreetperformance.com/psp/Sealit.html
if you want to try & hammer that area of the pan, i would use a 1/4" or 3/8" extension bit with a slightly rounded head before trying to use a sharp or even rounded over chisel. good luck whatever you do, oil leaks suck!