Hello there,
yes you can fit Corrado VR6 70ltr fuel tank into Corrado's with 4 cyl engine's and vice versa, the early Corrado's had the 55ltr fuel tank, then in 1992 the tank volume was increased to 70ltr.
The problem you're going to have is the fuel pressure, as the 16v as you already know has two fuel pumps, low pressure lift pump in tank roughly 3 bar of pressure, then high pressure fuel pump under car in swirl pot, which pumps at 5.2 to 5.6bar for KR 16v or 6.1 to 6.6 bar for 9A 16v, the 16v injection system is multipoint but is mechanical injection, final fuel delivery is via fuel distributor, which is controlled by fuel differential pressure governor/regulator, if the high pressure fuel pump doesn't deliver pressure as stated above, then the fuel injectors won't open as they need the correct specified pressure range.
The VR6 injection system including the 2.0 16v ABF only have the one fuel pump in fuel tank, pump pressure is roughly between 3 and 4 bar, this system again is multipoint injection but has electrically operated injectors, that receive signals via ECU.
I think you can possibly fit a MK3 Golf fuel tank into a MK2 Golf, but I've read on other forums the filler neck doesn't quite line up fully with fuel flap opening, but then others have said they fit fine, so regarding MK3 Golf tank into Corrado yes it may well be possible but how well it'll fit I'm not sure. But like I say you'll still need the high pressure fuel pump under car for your K-jet 16v injectors, unless you're able to fit high pressure pump in tank. MK3 Golf with VR6 (AAA) and 2.0 16v (ABF) I think use same fuel tank as the ABF fuel pump is very similar to VR6.
Corrado fuel tank
Part number 535 201 075H
Length 910mm
Width 630mm
Height 620mm
55 litre tank with narrow fuel neck 22mm for unleaded petrol
From 1988/89 to 1992 (weight 6 kg)
Corrado fuel tank
Part number 535 201 075AB / 535 201 075AE
Length 950mm
Width 630mm
Height 615mm
70 litre tank with narrow fuel neck 22mm for unleaded petrol.
From 1992/93 to 1995/96 (weight 9.5 kg)
Hope this helps
Si