1971 B Body 383 and 440 cars had a 5/16" vapor line that ran along side the passenger side frame rail and went directly over to the fifth nipple on the vapor tank , drivers side rear wheel well like you said
Along side that passenger side frame rail that steel line ended up front towards engine compartment- Then you had your rubber hose with crimp clamps , then a second steel line that ran up passenger side engine compartment inner fender and ended on top - Another rubber hose that ran over to the three nipple breather
Took hose off the three nipple breather
Then it was a matter of elimination
On mine , I disconnected the rubber hose at vapor tank - Got the air compressor fired up
And started pumping nozzle pressure 60-80 psi air pressure thru that steel line
Nothing on the other side wife said
Then I had to cut those crimp clamps bottom side passenger side frame rail up front
Got that rubber hose loose and off , was deteriorated inside
That’s where mine was plugged - Ran air again thru that steel line , came out other end , you could definitely hear it
New rubber hose on frame rail and new crimp clamps
Confirmed air flow with everything back together
Now to the vapor tank itself