I have a Platcat which I placed directly behind the passenger seat. It vents
between the back of the heater and the outside of the battery box--directly
through the floor. If you do this, be careful to avoid drilling into the gas
tank. Not a good idea.

Mine works well and provides more than enough heat on a cold night. I'll
make another suggestion you didn't ask for, but might profit by. I placed my
thermostat behind and above the driver's seat. I've learned this is not a
good location because it's effectively behind the direction of the heat
flow. Consequently, the thermostat underresponds to the heater and allows
the back of the van to get too hot. I'm going to move the thermostat to the
outside of the rear closet.

Good luck.
Dan in Madison
89 Westy 4 speed.