ive seen a number of burned vanagons.  not all from engine fires either.  but i can tell you that, yes, the plastic engine lid does burn/melt/cave in on top of the engine letting flames engulf the interior.  fiberglass burns too. seen many vans with fiberglass bumpers that were melted.   the safest thing is changing your fuel lines.  the metal engine lids are heavier, but in case of a disaster, they will buy you a little more time than a plastic lid, keeping the fire from entering the interior as fast as a plastic lid would.  the metal lid i have on my waterboxer van is from a 81 westy.  i ripped out that nasty mouse fuzz insulation they had underneath it and installed some of that racing style insulation - looks shiny like aluminum..it the kind that racecar drivers insulate their firewalls with for noise/heat insulation. i used the original wire springclips to hold it in place under the lid. plus i have the sofabed cushion and insulating pad on top of that.   so the noise is not bad at all.

just a fyi.
chris