Greetings,
By strict game play rules, the Field Gun infantry cannot be separated from their field gun. The entire unit represents a tow vehicle (jeep, small truck, etc) and the towed field piece. Most APCs do not have tow hitches and would not have the right size hitch if they did.
Think of loading a truck and trailer into the back of a C-130. That's how it would work.
This is for strict game play, on the map sheet rules. For between battles, what works for your game is what you should do.
Best,
Joel BC