If you had a "left over" 70 chassis IE early 71, "maybe" I don't know. I would lean more towards "you had it." It is important, however, to refer to the exact service manual, as they changed "year to year." I got a ticket (in part) for my non CA car (70 V code RR) which I was driving in CA in the early 70's. The eco Nazis call the shots, LOL
First look at the tank, filler tube, and gas cap
1....Is the filler cap a "pressure vacuum" cap? If so, you "had" the system
2....Inspect the filler tube in the trunk. Is there a 1/4 vent line factory welded into the filler, and doubling back down through the trunk gasket? No? Then you had the system
3.....Inspect the tank underneath. If there is ANY fittings on the tank besides the one for the fuel supply in the sender, you "had" the system
"How to fix"
The problem (you may have) is going to depend on "which tank" you have. Some of the systems had a can separate from the tank, a "liquid vapor separator." Some of these were built INTO the tank
If your tank does NOT have the built in vapor separator, then that vent line WILL siphon if the tank is full enough, and parked in such a way to start a siphon out that line
BEST way is to pull the filler neck out, weld a fitting onto the side, and duplicate the 69/ earlier vent, and plug the second line, or else use it for a "return" line
The early systems were simple........a line welded to the filler. The filler provided a "vacuum break" to prevent siphon. The 1/4" line came up high inside the rear quarter, and doubled back down through the floor...........where it simply ended.........inside the rear frame section
http://i55.tinypic.com/2cmugso.jpg
The 72/ 73 service manuals SEEM to suggest that A bodies do not use a separate vapor separator. But the 70 and 71 service manual shows this, below. Is there any evidence of this? Should have been a place in the trunk where the vapor separator can "was"