Ed, FWIW, I was SURE I had a rear main leak on my FE, but the primary source turned out to be something else. Over time, I found leaks coming from (in no particular order) the front plug (on side oilers), the rear intake-to-block seal (particularly at the outside corners where the head, block and intake all come together), and valve covers. Would it be worth wiping the motor down real good with brake cleaner and putting in some flourescent tracer dye in the
oil? That was helpful for me on a particularly pesky leak...obviously not necessary if you're positive it is the rear main. I hate to say it, but it could be that the repair was faulty...it has happened to me on FE rear mains, they're not the easiest seal to fix.