Weight savings is key though! It's still much cheaper than building the engine!
I probably wouldn't do knobby tires because I primarily focus on dragging knee on this bike
How about cutting a part out of your seat so it hugs the tank bolt? That way you could have the seat sit directly on your frame without having the gap. You'd only need to cut a semicircle out, shouldn't be too much trouble and I think it would look best. Also your seat padding would still fall over it so that would look nice
My thought is your new seat should butt up against the tank. I think you can gain clearance by lowering the tank mount bolt. Remove the rubber cushion and any other washers and grind the bolt head by 70%. As a better alternative, If you have a quality fastener store nearby maybe you can source a countersunk philips head bolt to gain even more clearance to allow the seat to slide forward.