You should remove the plugs and squirt some light oil in there before you (gently) try to turn the engine through by hand. Concerns are rusty cylinderwalls & rings and that there could be a buildup of fluffy carbon and corroded piston/head matter that'd jam up at TDC (the latter can usually be blown out with air). There's a good chance that there's water in the bottom of the sump from condensation, drain the oil before you crank it over and get it all through the engine. Once it turns over freely by hand put some fresh oil in and crank it on the starter with the plugs out until you get some oil pressure (don't overheat the starter, give it cooldown time between ~20sec uses). The gas in the carb has probably turned to varnish and evaporated and the inside of the carb could be corroded badly - to be on the safe side I'd take off the top and look.
You should check that all of the valves have lash before trying to fire it up; running for at least a few minutes with them intentionally loose (~.008-.012") can help clean rust off of the valve faces and seats.
Is this an original `58 1200 with the integral generator pedestal or a `61-up "40HP" 1200?