There are a number of posts here about preparing to start a long-idled old Chevy. I'd spend a few hours using the search function in this chat room and reading what others say about it. One of the posts is here, but there are many others like it:
I recommend that you proceed as follows (assuming that the engine is in the car):
-- Remove oil pan. Check inside of crankcase for cleanliness, rust, debris, moisture, etc.
--Using a small mirror and flashlight, check that oil galleries above main bearings are clean and free of debris. Examine cylinder walls to confirm there is no rust and that they are free of scoring and wear.
-- Remove one rod bearing cap, being careful to note the position of the shims so they can go back in the same places. Inspect rod journal and bearing. If they're new/resurfaced, it will be obvious. Reassemble and torque to correct torque, reinstall cotter pins.
-- Same drill with one main bearing, looking for condition and cleanliness.
-- While oil pan is off, lubricate cylinder walls, rod and main bearings, piston pins, camshaft bearings, cam lobes and lifters with clean motor oil, reinstall oil pan with new gasket.
-- Remove side cover, inspect push rods and lifters and check for rust/cleanliness. Lubricate lifters, reinstall side cover with new gaskets as needed.
-- Remove valve cover. Check for cleanliness, rust, etc. Lubricate rocker arms, valve stems and push rods with Marvel Mystery Oil (MMO). Leave cover off for now.
-- Remove spark plugs and squirt one teaspoonful of MMO into each cylinder. This will act as a penetrating oil and both free up any sticky rings and lubricate the pistons for startup.
-- Check that plugs are clean, reinstall.
-- With engine either in car or in test stand, lubricate starter, generator and water pump, check fan belt tension, etc.
-- Fill engine to full mark on dipstick with good quality oil, ensure radiator is topped off.
-- Remove distributor and using a slotted piece of 1/2" round stock to engage the oil pump, turn oil pump with an electric drill until oil pressure is raised to normal. Continue running oil pump with drill until oil is observed seeping from all rocker arms. Run the oil pump for at least five minutes to fill all the galleries and oil reservoirs with oil. Reinstall distributor and reset timing.
-- Crank the engine over slowly by hand and check that all rocker arms and valves are operating. Statically check the valve clearances. Reinstall valve cover.
-- Prime the fuel system to fill the carburetor bowl with fuel.
-- You're now ready for a short test run. (Don't be alarmed if there is heavy smoke for the first ten seconds on startup -- that's the MMO burning off). If the engine really is freshly rebuilt, you don't want to rev it up too far above idle or to run it for more than a minute or so the first time. The new bearings can generate a lot of heat until they wear in a bit. The idea is to run it for a minute or so, shut down, check for leaks/problems, wait an hour or so, start again and run a bit longer, shut down/cool down, etc. After 2-3 short runs like that, you can extend them and then begin driving the car gently. The main point is to avoid overheating the engine when it's brand new.
Sorry to rattle on for so long, but I hope that's the info you're looking for. One final recommendation: On the day you plan to start it up, find someone in your area who has done this before and ask them to stop by and give you a hand -- you can learn a lot from the voice of experience. Good luck with it!
All the Best,
"It's wise to choose a SIX"