Mine is an 80 litre where the spare wheel should be. You can get larger ones fitted inside.
Incidently, I've been reading good things about a local installer.
LPG 4cyl OMVL /AC Multipoint Conversion in South Wales! on eBay, also, Other Cars, Cars, Cars, Parts Vehicles (end time 15-Jun-08 10:00:00 BST) People seem to travel from all over the UK to get to him. Only £1000 for a 4 cylinder and £1200 for a 6 cylinder. It's the same kit as is on mine which I believe Ian paid £1500 for, just for the parts.
I'm going to go see him about the misfire I currently have.