I covered the gap with the seat on this bike with a seat otherwise similar to yours and the same tank. I think the hump idea is fine that you have going, but it does cause you to have to make a nice joint at the tank and an additional transition at the seat where the foam is. It's a good idea to at least mock up your foam so you can get an idea of the finished lines of the bike before you invest too much time in styling you might want to change if you didn't have so much in it already. If you are going for a more rearward looking seat, an RD400 tank is about an inch longer and a fairly easy swap (holds more gas too, a real asset on an RD!). An inch does not sound like much, but visually it makes the tank seem a lot longer and the solo seat a lot shorter than you would think from an inch.