Dave, I'll give a more detailed response later when not on my phone but...
Single hole tank will be perfectly adequate for a 2.2. You can get a 400mm diameter, 100L tank in but it's a squeeze and heavy... A 80L will be sufficient and should give 300+ miles between fills.
I think there's a 2.2 manifold floating about on the forum somewhere for that purpose.
There is, i have it in my garage. The idea is you covert that one to be LPG, yours becomes spare when you do the fitting and goes to the next person who wants to convert a 2.2, drop me PM if you want this

I've got a 90L torpedo tank in my saloon, its rotated so that it over-fills ever so slightly, but I usually fill up at Asda at High Wycombe as its just 58p a litre, so by the time I either get to my parents or my flat, i've used some, I can get about 74L in from empty, this gets me 350-400 miles.
Single hole is fine, you don't need the bigger flow unlike the V6. Also very few pumps take advantage of a 4 hole pressure, takes about twice as long to fill up on LPG than petrol i've found, much to queuing petrol drivers annoyance!
