Granted it has been a while since I worked on a 6-71, but I think I would do the following at a
minimum before trying to start it & then drive it any distance.
1) Roll the engine over by hand to ensure it is free to turn. Open the air box covers and look for moisture, stuck rings, or signs of damage in the cylinders. Clean the air box drains.
2) Remove all six injectors and have them bench tested or just replace with rebuilt units. A stuck injector could lead to blowing a tip and catastrophic cylinder failure.
3) Flush the fuel system & replace all filters. Drain the fuel tank and refill with fresh fuel. Carefully inspect the fuel lines (supply & return) replace as necessary
4) Drain and replace engine oil and replace the filter(s).
5) Drain the cooling system and replace the coolant, replace the thermostat. I would strongly recommend replacing all hoses in the system. Clean the radiator fins for debris. Consider having the radiator flushed.
6) Replace all fan belts.
7) Check tranny & re-end oil levels.

Lubricate all throttle linkages and ensure proper movement.
9) Replace the air filter cartridge or clean the oil bath (if equipped)
10) Check the blower screen for debris or deterioration.
Once the engine has been started and is deemed to be running ok, drive it and get it to operating temps for 25-50 miles and change the oil and filters again. I would consider changing those oils once they are up to operating temperatures. This would help get any moisture out.
My biggest concern would be accumulated moisture and any damage that it may have cause. I'd plan on a lot of new leaks due to gaskets drying out. (not that a 6-71 ever leaks, LOL)
Keep up posted on how it goes.
Andy