It would probably be great fun to show up with a 427ci engine in a VW Rabbit p'up.
Currently if you want to run over "C" displacement in a minitruck, you will need to go diesel. You can run over 1200ci if you so desire, (and can find a good place to sit on top of the engine), since there is no requirement that it be an automotive-based powerplant. The only restriction is that it is representative of what is sold at dealerships as diesel pickups. Ship engines in minitrucks are usually in the very back of the dealer lots, so you need to look carefully.