To gravity discharge, the entire tank must be well above waterline...so I don't THINK a tank shaped to fit the hull
under the v-berth will be able to do that. Nor do I think there's enough space under the v-berth for a tank that big...remember, you have to allow space for the fittings and hoses.
Plus, as John said, it would add a lot weight to that part of the boat--at least 300 lbs plus the weight of the tank...and a FG tank that size would be heavy!
You might want to consider a Plan B: A smaller plastic (rotomolded PE) tank. And your best source is Ronco Plastics Ronco Plastics marine Tanks
They make TOP quality thick-walled water
and waste tanks
for a very reasonable price
and have more than 400 shapes and sizes, over 100 of which are non-rectangular, and they install fittings in the sizes and locations specified by the customer when they make the tank.
However the toilet has nothing to do with emptying the tank..it only sends bowl contents TO the tank. So the type of toilet is immaterial as long as it's capable of getting the waste into the tank.