On board artillery spacing

[AlienXXX]

I would normally have 3-4 hexes space between arty units.
If ammo re-supply is allowed an ammo truck would sit next to a gun and then, in time, move over to re-supply the next gun, and so on.

For WW2 I do not think it is realistic to have more spacing between arty units. In wooded areas I guess they should be closer. Ideally, guns should be in view of at lest another unit of the same squad.

I would place trucks far enough that they don't get blasted by CB fire. But close enough they can get to the gun position within 1 turn.

The existence of transport or not for each gun/mortar should be agreed as part of the rules by both players.
In a meeting engagement it is assumed both forces are mobile, so transport for arty (or even for all units) should be required.
In an "advance" or "assault" the player being attacked could have one or more fixed positions. I would not expect guns placed in fortifications to move around during the time of an assault. For these cases, transport should not be allowed.
However, for the purposes of a player vs player battle in SP, it may be unfair to forbid the player being assaulted from moving his guns away from CB fire. Realism a side, I guess that the arty transport should be agreed between players before game.