On my '66 and the '67 I had the exhaust exited behind the right rear tire. These were window vans though. You might do a google search for images from original sales literature.
thanks have been looking but havent found a good photo yet, by the way, my local muffler shop says they came both ways, in front and behind the rear tire
My 66 and 67 cargo's had tailpipe exit out the back passenger under the bumper. Once it reached bumper it tipped down slightly. Not sure if yours would be the same, but these both looked to be original and both were identical. But after all these years it's anybodies guess as to being original factory design.
That where mine is on my pickup. Comes out right under the D on the tailgate.
On a van, if it has pop out windows in the rear you should never run with them open regardless of where the exhaust exits. The exhaust can get caught in the air behind the van and sucked in the rear windows. And, I wouldn't want my exhaust to exit below a side pop out. Could drift up and inside while stopped. Just my thoughts.
My window van exits on the passenger side, under the bumper with an angle to the outside of the van. I don't know if it's original, nor if it's even relevant to your van. Just additional information for ya.