I never looked into that, but I do see some pretty nice hardware in private hands....so well see.
Might be able to register it with the BATFE as a destructive device/and or manufacture sample.
76MM HELLCAT TANK DESTROYER! - YouTube
For example this 76MM seems to be functioning pretty well.
90mm T8 Cannon Tears a hole in a hillside - YouTube
So does this 90mm
I think its more of a question of how much you want to spend, because this area of military collecting is for very rich men. Ted for example who seems to have a few bucks can't afford to play in this field. Although he did mount an MG42 on his Ferrari...so does that count as an Italian army mechanized support vehicle?