Just in case there are things running all around willy-nilly, there actually IS a differential vent that runs up into the frame. They changed the tank layout on the '77's, so might also have changed the location where the diff vent was clamped to the frame. Maybe someone with a '77 will post up a pic.
For the tank, the charcoal canister really is beneficial. Your '77's filler neck will only accept non-vented caps anyway, so the canister does multiple duties.
But the '76 and '77 setups were also the simplest, so you should be able to set something up easily.
Is there still a steel line clamped to the passenger side frame rail running up to the engine compartment? If so then all you need to do is connect the vent on the gas tank to that steel line, then add a charcoal canister up in the engine compartment.
The style of canister that was used on your '77 is also very easy to find brand new from many sources. Most of them look identical on the outside, but only some have the actual tab for screwing it into the firewall. If you look over on the passenger side of the firewall, you might see a small rib of metal welded to the firewall and a small screw hole in the very top lip/pinch-weld area of the firewall.
The canister sits on the rib and a plastic tab on the top of the canters screws to the hole on the top ridge.
The plumbing runs like this:
1. The small hose fitting on the canister attaches to the vent tube/hose from the fuel tank.
2. The mid-size fitting on the canister is for a float bowl vent, if your carburetor still has one. If the carb was swapped for one without a bowl vent, or if you are running EFI you just cap off the mid-size port.
3. One of the large ports connects to the air filter housing so the fumes can be injested into the engine. But NOT intake vacuum! Just gentle vacuum from the air cleaner side of the carburetor.
4. The second large port is covered (but not closed off) with what we call a "mushroom cap" that lets it breathe, but keeps debris out.
And that's it. For a carbureted engine anyway.
You did not say if you're running EFI or not, but if normal (non-factory functions) EFI you obviously will not need the bowl vent, but you still run the line from the mid-size port to the air cleaner housing.
If on the other hand you're running full factory setup with the computer controlling what's called the "CAN-P" (for canister purge) circuit, you will cover both of the large ports with mushroom caps and use the mid-size port for your purge solenoid to a full manifold vacuum source.
This is the only condition that you would run the evap system to manifold vacuum because it's all controlled by the computer.
Hope that helps.
Paul