As for spaces, I thought all the ammo took up only one space?
A picture should help
This is a original Hetzer model found on sarna.net
Next to a APC Italian? found in 3d warehouse
The Hetzer got a 150mm cannon and 4tons of ammo. And I did not even use the magical 10round magical.
Vehicle space is abstract on one hand it says how big a vehicle can become. A 5t ammo load on a truck might not be a big issue. But when you add armor or excessive ammo to the mix things become interesting.
In contrast to infantry space ammunition can be stored in every tiny room. How the ammo got to the gun is secondary.
A Hetzer based on a truck would look more like the Dana, Archer or G6.
4tons stored but the space increases the vehicle.
It's not exactly rocket science but in the end a design should feel ok