@Frenchy quote[I've found this one in a book about the DAK, but you can find it online as well, posted by...."Panzer Bob" ]quote
Yeah. I'd love to take credit for this one, too! Of course, That's also the case whenever I pass a "Bob's Big Boy Restaurant" over here! Somehow I never seem to get a franchise check from them!
@Mike;
Just keep it straight-forward and don't try making up some fancy mount! The whole gun and its standard mount would have gone into that lorry bed just fine, and if you keep things looking like a crew just took advantage of the available truck, I doubt anyone could call you on it with a straight face!

It really IS plausible that a DAK crew could have done just this - they mounted Breda guns, Flak 30 and 38 guns, captured Bofors guns - whatever they could find - on all sorts of available wheels - German, Italian, Commonwealth, etc.
I might suggest that this could be an opportunistic Luftwaffe field unit "trick", as many of these 3.7cm guns were manned by Luftwaffe flak troops around airfields and seconded to army formations... Just an idea

In this case, one might consider having the gun painted a Luftwaffe scheme and simply "plopped" into the truck, which could be a British scheme with German marking, or...
I think your build-lamp is GREEN!
Bob