Hi DS
You sure seem to be struggling with this one first what van have you got.
I've got an ldv high load and boarded out behind my seat to head height the only gap is small hole for the side sliding door runner. With storage compartment above my head so basically the cab is sealed from the van. It also has a rotating vent in the roof.
I've run the machine recently for nine hours only stopping to refuel back doors shut together enough for hoses about 6in centre gap had very little fumes in cab.
LPG conversion kits on eBay
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&ssPageName=STRK:MEWAX:IT&item=360115409757Their main site with a PDF fitting instructions LPT 3 is for gx390.
http://www.pump.co.uk/shop?code=LPG1Gas bottles are available in the camping shops some higher shops the old-fashioned DIY stores roofing companies.
Alternately you can use a refillable bottle which costs more but you can refill at the local petrol station will cost you less to refill then exchanged bottles
The link below should give you information and your local petrol garage with refillable gas facilities.
http://www.gasrefill.com/Why_choose_refillable.htmlHope this helps
All the best
Mark