Brett...if you have the charcoal canister under the hood, then ya should have the so-called "condenser" up inside the driver-side rear fender that is also part of the evaporative emissions system. Those systems were used on many/all vehicles prior to the evolution of what we refer to around here as the "emissions motors", etc. You May be "pre-smog" as pertaining to your local/state regulations, but ya still have an eec system to deal with.
And...your tank will look very similar to the one I posted regarding an oem 19 gallon tank from that '73 sii that we converted to the 30+ gallon tank. All this guapo is covered in this current thread.
Look up behind the driver side rear wheel at the side of your fuel tank.. Determine how many hoses/fittings you can see and compare with the pics I posted.
Also, many sii came with a molded plastic filler neck that cracked/distorted after a few years of service. That will allow fuel to escape, as will a rotten connector coupling.
I'd pull the tank and all the associated plumbing out before ta start dealing with the floors/sills/rockers. Lay it all out and see whatcha got and then determine what to do about it. That pic Craig posted tells the whole story, very nice labeling job Craig!
I'm attaching again, a .pdf of the evaporative emissions system from an ihc service reference, first diagram izza sii. This illustrates the big picture.
I know what I'd do...but I won't discuss it here on the forum, email me privately for my "opinion"!
So is this gonna be the winter project once the snow is piled in front of the garage door and ya can't git the rides outside??? I've spent new years on the cape and in new hampshire on the same swing, so I know all about "winter projects" in yore kuntry! Ya got the antifreeze in tha matchless???