i'm not 100% sure but i don't think having gas is going to be in your favour if you want injection. i was under the impression that the tanks they use are different. so they have to rip out your old tank, put a new one it. maybe the gas lines are about the only thing that will actually be the same, but chances are they are different too.
tbh i'd stick with the straight gas. the engine will run smoother on it than petrol anyway, and you might gain a few kw (i'm talking 5 max, my vn v6 had ~4kw difference between petrol and gas and that was a crappy duel fuel setup) by taking it out, but it's not like switching back to petrol is going to suddenly make the car a massive beast...
btw, if you do go injection, you want LIQUID gas injection (not vapour). vapour = same power as petrol. liquid = more power than petrol. BUT liquid systems run sequential injection, while the VQ runs groupfire, so modification is required (unsure of the specifics). i talked good and hard to a gas installer about this once, he recommended the carby setups for a 5l. as the carby setups do quite well, especially when you start talking twin carbys/converters if you need good power. you probably wouldn't need to modify the tank, but then again you might need twin tanks depending on how much power you're looking for...