LPG is used in petrol cars, the engine usues it instead of petrol, its half the price per litre and theres no performance difference really (not noticeablle) only thing is you have the extra tank in the boot so you loose boot space
If you use a torodial (round) tank in the spare wheel recess you don't lose much room but i had a huge 90ltr one that went accross the rear of the back seats as i did a lot of travelling from Lancs to Glasgow. (£20 there and most of the way back)
The rear seats when i measured mine looked like the same size but never getting round to fitting the rear you would have to take a lot of measurments yourself to be sure.
Good luck.
Best thing to do would be to measure the subframe and it's locating pins and holes then do the same with the GTV if there the same subframe then no hassle a straight swop.
If you've seen some in a scrap yard just take the measurments along and check them or ask some of the guys on the Alfaowners.com site