you are planning to cut the floorpan and assuming it will pick up air and simply cool the radiator?
the cooling potential of the radiator is only seen with a differential of air pressure across the fins, lower pressure at the back and higher at the front will cause air flow, Engine rooms of cars exhaust air underneath it whilst moving forwards, a lower lip adds to this vacuuming out effect and inturn more air is drawn through the radiator - infact if you look at older mercedes benz cars they had massively huge belly pans with small louvres cut into them to exhaust the engine bay as the car moved on the road and also thermostatic side louvers in the wheel wells to exhaust "stalled hot air" from the engine room if the car would sit in traffic.
the VW engine bay design is such that it fills with air from the upper vents, The original air cooling system used this fresh air and pushed it with the fan onto the cylinder barrels, heads and oil cooler with use of proper shrouding and actually sealing off the engine bay with rubber gaskets.
By your installing of the radiator in that place you will stall out the air movement and most probably will flood the car with hot exhausting air.
Do the front setup and live in peace - you will need stainless steel pipes and the bleed valves - also drill a bleed on the engine top hose pipe barb fitting, a smaller water pump pulley and a remote mounted pressure tank to keep the water level above the radiator and engine.
