You might consider watching a couple of Vice Grip Garage's revivals of squarebodies on youtube. He's native to small block chevys and talks through a lot of the stuff you have to check on a vehicle that has been stored... I don't know how many SBCs he has done but you cuold probably find 2-3 squarebody videos of his (Derek's) and that would give you the confidence that you aren't going to do something totally wrong. And, to be clear, he often does things the total backyard way, but hey...it's a show.
She got oil? Is the oil real or half water? (change the oil)
She got coolant? (fill coolant at least until you can see it 6-8 inches down in the radiator)
Hoses intact (upper, lower radiator hoses, heater hoses)
Wiring generally intact or eaten by mice? Do you have the big 4-5 wires necessary to run the motor? (Ignition, starter cable, ground cable, coil wiring, honestly, that's about it as far as running the motor on a 77.)
Are any of the vacuum hoses broken, cracked, or disconnected? If so, just ensure the carb and manifold vacuum ports are capped before trying to start. One a lot of people miss on that Hollley is the big gulp vacuum fitting on the back of the base of the carb.
Is the distributor cap and rotor clean?
Like the advice on a drip or two of oil in each cylinder... while doing that make sure the plugs are either new (cheap), or not broken, cracked, or fouled.
Are the spark plug wires intact? A few minutes with an Ohm meter can check the wires aren't bad. While you do that (one by one), check that they firing order is correct. A lot of cars that are parked get fiddled with at some point and plug wires is one spot to double check.
I'm assuming in 77 it was HEI, so no points to check, but if it's got a holley it's modded, so check to make sure it doesn't have points and if so you may need to file the points to get her to fire.
Battery good? Of course not, but make sure you make good connections to the battery when you put it in there.
The advice to take the distributor out and prime the motor oil while turning the crank a couple revolutions is great and very careful. I would at least ensure it turns a full two revolutions with a wrench on the crank (not the starter) before I hit the starter.
A very big area to watch out for will be rust in the fuel system. A truck sitting that long, even under cover, is bound to have a rusty fuel tank. A FIRST step before starting it would be to get a cheap 3/8 fuel filter and put it in the rubber fuel hose that goes between the frame rail steel one to the fuel pump on the lower passenger side of the truck.
Bonus is you can see whether the engine pumps fuel from the tank if you use a cheap plastic filter that you can see thru. A more conservative approach would be to attach a cheap electric fuel pump to the steel line and try to pump anything that is in that fuel tank out into a bucket before putting new fuel in. I'm not sure how much liquid you would have in there. a truly conservative approach would be to just plan on putting a new fuel tank and sender in there.