On that kit, the seam is what builds character.
I just looked at mine and something that always bugged me is the vertical scribed lines, there's way too many of them.
Anyway, have you considered brewing your own "sprue goo"? Get a fresh bottle of Tamiya Extra Thin glue then chop up all the sprues that came in the kit and add them to the bottle of glue. Keep adding sprue and stir up the goop until you get something with the consistency of putty. Use that as your filler and you won't have to deal with putty shrinking six months down the road or filler that dries harder than the plastic.
Another possibility is using Tamiya's white or gray primer in the square bottle. This is a fairly "hot" lacquer primer that has a consistency that's just short of Testor's tube glue. Make sure its fully cured before you start to sand it.
My other idea is to find Tamiya's plastic putty in the tube. I'm pretty sure this stuff is "sprue goo" made from white plastic. I had really good results using this stuff on the sleeper of an Italeri Peterbilt truck (these kits were infamous for having "short shots" in the plastic).
Anyway, keep calm and keep sanding.
Those vertical scribed lines in the hull still bug me and I'd suggest finding pictures of the real article to make sure they are there.
