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Hi
a few more thoughts from my old memory.
Hasegawa do some 1/72 scale kits, which included jeep and 37mm a/t gun, and also
a 155mm Long Tom gun, also a High Speed artillery tractor though I don't know if they're still available.
Italeri used to do a 155mm sp gun in 1/72 scale.
There are also the old 1/76 scale Matchbox kits of M16 AA half tracks, a 155mm sp gun and the Priest.
In addition Plastic Soldier Company do a Loyd Carrier with 6 pounder a/t gun with parts for the US 57mm version.
Hope these thoughts help - when I was young I had the problem of finding US guns but they're out there in one form or another!
Best regards
Richard