Can't see the seat in your post, the image didn't come through on my end, so hard to say much about that specific one. For a 75/5 though, I'd lean toward something that keeps the long, flat stock profile, since the bike's lines are so simple that a lumpy or overly sculpted seat tends to stand out. Thin pan, modest padding, and a bit of a kick-up at the back looks right with that fender you've fitted.
If you can repost the pic or tell us who makes it and whether it's a stock-style or custom piece, people can give you a better opinion. Also worth checking the seat hinge and the clearance over the rear fender and shock tops before you commit.