b4 b3 fuel tank swop

[golfmad]

hi guys im here again and hit a snag ive put my b4 tdi into my b3 petrol ive cum to swop the tanks and found b3 is taller and b4 slimmer tryed to fit b4 in but straps wont go round it but the problem i have if i fit b3 tank back in i cant put diesel in as filler neck has a flap on it and b4 one dont any ideas guys.plus the b3 petrol tank has the vr6 pump in it still which some one on here said i can use it .its ok useing it but its to power full keeps leaking diesel out the leak off pipes and there brand new leak of pipes .not sure what to do.