Unless it has total seal rings ( gapless ), you will always have a leak at each cylinder. What you are looking for is one that is way different than the others.
I would run it on a stand then checking it.
Rebuilding it then storing it is not something I would do. Valve springs and rings don't like to set under compression, seals dry out when not used, and no matter how good you seal it up or lubricate it, it will have some crude build up or rust from setting. Your best bet is to run it on a stand, make sure theres no bad sounds, it has oil pressure, and doesn't over heat. Back off the lash adjustment, change out the oil, over fill it a lot so most of the bottom end is under oil, pull the plugs so the cylinders dry out, and drain all water out when its still hot. Let the heat of the block dry it out. Don't use coolant, water drys much quicker. Once completely cooled off, pour some oil in each cylinder, rotate the crank to spread it around, install spark plugs, wrap it up and put it in a dry location. Once every six months or so, roll it out and turn it up side down to coat the whole inside with oil again, and put it back away. If it was rebuilt and you all of a sudden needed to use it, you would still have to tear into it to check for rust or mice, then hope the rings didn't get stuck and the valve springs are still good. Always, store engines with valve adjustments back off so valves are closed, pressure is off the spring, and hydraulic lifters are released. Use good rubber plugs on all holes ( not tape !!! ) and make sure they seal. It wouldn't hurt to include a box of those small moisture absorbing packets scattered around the engine before wrapping it up, the more the better. Wrap it tight to keep out bugs, wasps really like engines. If the carburetor is still on it, spray it down with WD40 inside and out, then plan on rebuilding it when it's time to use it. Or rebuild it now using WD40 on the gaskets and rubber parts, vacuum seal it in a heavy plastic bag, and store where it won't get damaged.
