That is really strange.
I'm looking right at my 1972 Owners manual on page 70 and it's pretty darn straight forward. 20 US gallons = 16.75 Imperial (Canadian) gallons.
Naw, couldn't be that. And it couldn't be a wagon tank because they are 23 gallons and the filler spout is different.
I'd be taking a close look underneath to see what the tank looks like. If you're using this baby for road trips, (and assuming you're cruising along at about 70-75 miles per hour) you darn right you need the extra 4 gallons.
If it's not the right tank, you can probably get a proper one for just around 200. Definitely worth the peace of mind. 
Is this car stock or a resto mod? Might explain the difference. Anyway, keep in touch let us know what you find.
Allan R