The 3" M5 was actually an AT gun, not an artillery howitzer. It was the same caliber as Sherman tanks, 76.2mm. It was usually pulled by a halftrack, M3 or M5. I guess a 2 1/2 ton truck could pull it.
Since the ammo is basically the same as a Sherman, any Sherman ammo would work fine on it. 105mm ammo boxes will be a bit too big.
DToys makes a set of five boxes w/decals for them.
AFV Club does just brass rounds, w/decals.
Verlinden used to do a set of rounds and boxes. You may be able to still find them.
A couple boxes came in the Academy AFV Accessory set 1.
As to the 3/4 ton Dodge WC series, the best is the AFV Club or new Zvezda WC51.
The Zvezda one looks to be pulling a Russian 76mm Field Gun.
The Roden one is a Vietnam era M37. It is actually a pretty good kit.
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