Mike,

you can remove the fire wall to get access to the fuel tank. Note that on
my '75 there were an additional 2 screws to be accessed from underneath
the bus; they weren't mentioned in Bentley and I couldn't figure out why
the firewall wouldn't come out at first...

With the firewall removed, you can access the vent lines above the tank. 
At various locations those metal lines are connected to each other by 3
inch long rubber fuel hoses. You want to replace them all, including those
in the engine compartment (there's also one hidden behind the spare wheel
well).

While the firewall is off, check to see whether the large hose from the
filler neck to the tank is in good shape. For even better access to this
part, pry out the black round plate on the right of the engine compartment
and remove the ECU. With a full tank, rock the van and see if the large
hose is still dry. 

>   '78 Westy ("Yosemite Sam")

Hey, wait, I had a trademark on that! ;) My Westy has the chartreuse paint
job, aka Yosemite Yellow, and my name is Sami, so Yosemite Sam is my CB
handle. 

Sami