no you can't without cutting the boot floor out as there won't be enough ground clearance to get a big enough tank in, legal minimum clearance to ground is 240mm. the shape is also the wrong size to get a twin tank in, i have tried.
The largest single tank you could fit without cutting the floor is around 45ltrs water capacity, so 80% fill = 7.9 gal approx, not enough really, the smallest range i would allow on a car is 200miles and would refuse to fit a kit if it's gonna have less range than this for the road, for the track it's obviously not so important but 200miles is still the range i would aim for.
With the floor cut out i have fitted a 70ltr tank, 80% fill = 12.3 gal approx, @25mpg thats 308miles for £39.20, @40mpg it's 493 miles for the same price assuming around 70p/ltr for the lpg, not to be sniffed at is it?
I am looking to make replacement boot floor sections so you would cut out the floor weld in the frames i supply then weld in the replacement boot floor, obviously this if quite a lot of work if the car doesn't do a great deal of miles.
On a normal conversion where economy is the priority around 10k miles is the break even point, however on our gtir's the break even point would probably be around 5k miles.
I ran an sr20det on a vapour kit when i had the rolling road and made 220hp @4000rpm @4psi with everything else standard, unrelated engine problems stopped the testing but the results were very promising.
I ran an sr20de on the same kit and that made 170hp all day long.
Liquid injection is the way ahead for high performance and i would only fit a vapour kit on a road car/van/motorhome. the biggest factor after cost is lpg's resistance to det.