Oddly enough I was looking at an online quote for mine this aft. As your van will be carb rather than injection the price will not be the same but I doubt if it will be far out.
I was quoted £1667.50 I know that the actual cost would be more than this as this quote was for a Boxer van. We have water tanks under our vans so the fitting of the LPG tank would not be as straightforward as they quote for.
Some time ago I did a spreadsheet for "my" club to work out the cost of payback on a gas conversion. Based on the above figure and an average of 20 mpg, 7000 miles per year and £1 per litre petrol 57p per litre gas, I got the following figures:
Using 100% gas, the break even point is at 22300 miles or 3.19 years, 75% gas gives 29733 miles /4.25 years and 50% gas = 44600 miles / 6.37 years assuming prices stay as they are now - which they wont.
HTH
Frank