I replied earlier when I had to get out of the house quick, didn't give all the info.
Bulkheads: I got them off of Amazon, not much $$$. Installing, even in a closed barrel, is a cinch.
Spigots: The bulkhead "output" has inside and outside threads. I recommend going with a spigot that uses the outside threads. My barrels have a 3/4" spigot, filling a watering can takes a good amount of time. There's head pressure from the amount of water in the barrel, but not enough to force water through quickly. I plan on hooking them up to a drip feeder, to water an approx 100sqft terraced veg garden.
Overflow: Damn. Go big. I got these in place a day before our first real thunderstorm of the season (Central Massachusetts), the barrels filled up in 15 minutes. Overflowed quickly. This on a 1000sqft ranch, 3 55gal barrels on the front of the house. Got 4 days of rain expected starting tomorrow, need to quickly get some improvements in place.
One more thing. 55 gallons sounds like a lot. Drained the barrels today watering the veg garden, realized it's not so much. Eventually I want to put a couple of 275 gallon holders in the back yard, fed from the back side of the roof and overflow from the front yard. Maybe three of them. Then solar powered pump to feed back up to the front gardens.