I think if that is going to be the final seat shape then I would keep the fender. It would be nice if you could "bob" it a bit though
My Hodaka just didn't look right without a rear fender of some type so I'm in the process of putting a chopped one on there.
Mine is going to be in-set/recessed into the rear tail section so it doesn't sit below the tail,but will appear(most of it) to come out of tail section instead.
My fender is chrome and will only stick out about 6-7" and is mostly straight with just a slight down curve.
I like the look of the old Manx racers style bikes and the vast majority of them have at least some sort of rear mud guard on them even if they and radically chopped like mine. The size,mounting angle,radius,color etc. all affect how it looks when it's done and little changes can change the whole look of the fender. ALWAYS mock up the fender with the rear wheel/tire on so you get an idea of how to mount/shape it etc.
Look at the site below to see what I mean about the Norton Manx bikes.
CAUTION: The bikes on the site below are absolutely STUNNING and could induce involuntary drooling. I take NO responsibility if said drool does any type of damage to your computer and or key board.
Also I would make the seat rails/side so they cover or sit over the seat/frame rails. Covered rails look cleaner than exposed rails on most bikes. Just depends on the bike and style of racer your trying to go for.