Is it worth it - Yes , i did it on my 88 in aril 08 , paid for itself within a year (4.2k exactly) ...stage 1 's drink more so could be quicker (but will need bigger more expensive vaporiser so may not be)
Mine was a DIY kit, cost me about £450IIRC , and £25 to have it inspected and certified (your insurance company will want this)
cost will increase quite allot if you want twin tanks etc as does complexity of fitting
forget little tanks, they hold 80% of the quoted size (to allow expansion) so work out whats an acceptable range (on my 88 ive a 90 litre tank, so 75 acutely which gives 300-330 miles ...which i find ok, anything under 250 i find inconvenient (parents RRC does 270 and that's borderline annoying)
I believe the tax level is fixed until 2012 (info from tinley tech website)
Id suggest getting a closed loop (lambda controlled) system, costs about £100 more in kit form, but mine is worth 2mpg, and it has more power , especialy as you use the fuel, the fixed setting system i fund ok so long as the tank was kept about 1/2 full
Tanks vary in cost, single holes ones beeing cheaper than the 4 hole type, they are slower to fill (aparently) and more fiddly with the piping, but ive never had a problem, speed to fill varys on the pump more, so i wouldnt pay more
the more complex you get with tanks and there positions its going to cost more
the only way to work out the repayment, is to find the kit you want, work out its cost and then how many miles to cover it
(fuel consumption may be slightly down ...though i cant tell on my 88)
if you do it yourself it will take 3 or 4 days to do the first time, while you work out where to put everything, where to run pipes etc
I always run on petrol at the start and end of a journey in winter, economy is horrible on gas for first 1/2 mile or so. I cant see an issue with not using it though , in summer i dont, and my carb never has a problem with it (with injection systems its a good idea to run on petrol to stop the injectors sticking)
I had to do no mods to make it run on a carb engine (other than put the stop solenoid in the fuel line - might have to think about that one as the V8 has a recirculating fuel system ...cant be hard to sort though
Your ignition system needs to be very good, gas is harder to burn, so it will find any weaknesses (no need for upgrades though ...as some installers may tell you )
The engine runs soother and cleaner on gas than petrol
thank locations - a big one in the back is easiest , you could put one along the top of a wheel box , so to the side if you need the load bed
or you could get 2 under the seats - would need to make brackets and re-locate the battery though, and modify the seatbox
you could replace the petrol tank with one , and then have a petrol tank somewhere else (front like S2's and 88 S3's)
you realy need to alter the timing to run on gas, so it wants to be a all the time thing or not at all, as it will run badly on petrol when timed for gas and visa versa, a 50-50 compromise doesnt go that well on either
that answer your questions ? if not let me know and ill type some more