Did you happen to use ARP bolts on the mains? I had the same problem with my 418 stroker in my Dart. Kept having a main oil seal leak and pulled the engine three times before finding out that the ARP. main bolts were interfering with the oil pump sitting flush. Apparently it causes oil to shoot directly against the main seal and therefore causes it to leak. You have to clearance the oil pump a little bit to get it to set flush if you're using the ARP bolts which are a little taller on the head. After correcting the issue no more oil seal leaks. Just something to check