we've done some 1200 milers on my WG with a sissy bar backrest and a sundowner seat. wife is a bit short, so had to also change the pegs for some Kury Wings. No problems with the wife now.
course, went and spoiled her and picked up another bike, a '07 UC, so looks like future long trips on the WG are out.
Sundowner seat is a great seat and your wife will likely enjoy the ride much more if she is riding on one.
I agree with the above statments , I have the Sundowner on the streetbob and the iso wings from Kuryakyn. The wife is alot more comfortable , think the next thing Put on there for her is the 4" off set peg extenders from Kuryakyn. Just mt 2 cents worth