Make sure you have the right seal, first. Some B/W take the same seal as the 400 automatic, some take a different one.
Also, some seals may have a deeper outer shell than normal, and you can have the seal installed all the way, but it looks like it needs to go in another 1/8" or so to be fully seated, because of it.
It can be a pain to get a seal knocked on in the car, for sure. The seal drivers help, but most of them are so long it's hard to get a good swing at them.
As Ken says, just take your time, and walk it around, a little at a time. I have found that a body hammer with a flat head works best for seal installs...that's the only thing I use if I can't use a seal driver.
Adams' Apple2008-08-20 19:05:40______________
Joel Adams
C3VR Lifetime Member #56
My Link
(click for Texas-sized view!) NCRS
"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"