I have built many non leaking FE's. The only one I can recall having a leak for a short period was an SCJ in a '69 GT a few years ago. He bought a correct # crank that had some pitting in the rear seal spirals and was fine until he switched over to
synthetic. When he switched back , it eventually stopped leaking. Since I believe you purchased all new components, it would be a stab in the dark to diagnose from afar. Unlike what has been said at times, FEs aren't prone to leaking by design.