I have just installed a 292  y block out of a 64 truck.I had to change the flywheel andoil pan and front timing gear cover to fit in my 55 crown vic.After installing the engine I had an oil leak at the rear of engine.Ithought that the rear main was probably bad and replaced seal with new one ,I was expecting to find a rope type seal but it had arubber type seal.I carefully replaced the seal and it was not easy to replace the seal but I was careful not to damage it as i installed it.Put the side seals in and the nails ,finished installing pan and started engine to find that it is as bad now as it was before replacing the seal. I did notice that the engine doesn't have a vent for the crankcase except for the oil filler hole and thought that I would install a pvc system on it. My question is for anyone that has a lot of y block experience could there be too much pressure in the crankcase which might cause the seal to leak? I think that I am going to pull trans and flywheel and check  rear cam plug to see if it may be the cause of  the leak. What are your thoughts. I also plastigaged rear main brg while i had the oil pan off and  had 0ne and half thousands clearance. I knew that this engine had not ran in years but was almost new when truck it was in was parked.Anyone with simular experience. Thanks,veeee8

For a start you need to alter the crankcase venting. If the oil filler cap is plugged with dirt & oil residue, in effect the engine has no ventilation, which won't help your situation.

Paul