lol I forgot you 4.6 guys have the luxury of removing the entire back plate.
Did you use some new oil to put on the seal before sliding it over the crank?
Assuming you wiped any FM from the the crank, and used some fresh oil to coat the rubber part of the seal, Id say give it a few miles to seal up. Ive replaced several rear main seals over the years in the 5.0l engines and sometimes they leak for a few miles then quit.
The only time I have had one continue to leak is if someone before me had changed it and nicked the crank getting the old one out, or the spring in the seal had come out and worn a groove in the crank. Then its a matter of getting a rear main repair sleeve to go on the crank.