Ever replaced your tank? They LOVE to corrode at the base of the filler neck where it meets the tank, mud gets in between the pipe and tank then sets, then gets wet and sets over and over, rotting the tank and pipe. Could also be internal corrosion from low fuel over a long period.
I assume you have changed your fuel filter?
Have you checked your fuel pressure? It could be the Fuel pressure regulator. Think it's on the end of the fuel rail from memory with the vacuum pipe coming from it.
Also, your new fuel pump..... does it meet the min. flow rate of the G40? Ultimately it's only a 1.3 SC so getting an overpowered pump and manual fuel pressure regulator may be the way to go, overfuel the line and it'll just recirc back to tank. Having low flow rate could allow the qty of fuel (therefore pressure) to build in the line, but as soon as you go flat out, the injectors piss all the fuel into the engine but the pump cannot keep up.
Lastly, it may be the in tank lift pump. They're renowned for going wrong, the gauze can also get blocked. They can be quite fragile now with age so watch the plastic bits on it when you look at it. It's under the rear seat.