Carburetor leaks after sitting empty for long periods.

sounds like the fuel is drying the seals out and when they get wet again, they swell and seal up. Now I can accept that with leather gaskets but Holley metering block gaskets are very prone to shrink when they get dry. I opened up an old 1850 and the block gaskets looked like an hour glass with all 4 sides sucked in by 1/8 inch. Use some good blue gaskets on the Holley and maybe replace the gasket on the fuel pump.