Two possibilities that I have struck with these,
1.The rear main cap sometimes needs to be trimmed to clear the pan ( really the pan needs to be shaped to clear the cap- but its a more permanent fix to massage the cap ). While the pan fits , it distorts slightly just as you nip it up & the
oil getting thrown off the crank @ high RPM finds a way out making it look like the rear main leak.
2. There are more than one dia of seal for these cranks/blocks including one that barely contacts the seal surface, make sure you have the correct seal.