Before has the vent rollover valve under the car, near the right side of the cowl and the vent hose hooks to a tube soldered directly on the tank. After has a plastic rollover vent valve and grommet on top of the tank that the vent hose hooks to. My car is after (06/22/79 build date) and the trunk floor has a hump in the center for the vent valve. Other than the venting style, the tanks are the same.
The sending unit is the same for before and after (at least according to the '79 parts manual). There are two different senders, one for 49 state, one for N95 CA only. The unit Rock Auto has a photo of, is the CA only unit with a fuel return tube.
You can install the after tank. You'd need the vent valve and grommet. I ended up using a valve and grommet for a Jeep Cherokee of the mid '90's (IIRC). Did have to make a spacer washer about 1/16" thick to go between the valve and grommet to keep it tight. The Jeep tank hole is slightly thicker than an F/M/J tank is. Of course, you'll also need to make the "hump" in the trunk floor, if not there already. Haven't checked in a long time but, when I did all this 12 years ago, the correct vent valve was made from unobtainium.
If you go that route, the original vent valve can be removed but shouldn't hurt anything if it's left in place.
I see Rock Auto lists the before tanks. Shows the Spectre Premium as "only one remaining". There's also Dorman and some brand I've never heard of. My 1st choice of the 3 would be Spectre.
One thing. Get an NOS lock ring or a good original. The aftermarket ones aren't made right and don't hold the sender tight enough to the gasket, and it will most likely leak.
