Step 1: Acquire the Barrels
Step 2: Barrel Orientation
My thinking was that I just didn't trust the thickness of the sidewall for a tap in connection since I wouldn't be able to thread a nut on the backside, so I went with a connection into the bung plug, thus needed an upside down orientation.
This provides for the barrel to be completely emptied, unlike most with a side spiggot - but also required me to build the base platform to elevate them slightly for the plumbing.
Since they store on a second story I had no issues of elevation to accommodate a watering can either so the base was fairly routine to build as opposed to one 2' tall.
Step 3: Platform/Base
Step 4: Downspout Gutter Connection
Step 5: Barrel Input Connection
Step 6: Intake Filter
Step 7: Intake Filter Install
Step 8: Manifold System
Connections: The 3/4" PVC runs into a threaded fitting then into the bung plug (Pre-threaded) but you will have to drill through the bung with a 7/8" spade bit, careful not to damage the threads. I used teflon tape & pipe dope on these 2 connections (Bung plug / Barrel & the 3/4" threaded fitting / bung plug - (DO NOT EPOXY) You will need some rotation play in the pipe once you tighten the bung plug you will need to orient the pipe out to the front of the barrels. If you epoxied the connection between the fitting & bung it is unlikely your pipe will be in the exact position needed.
The dual ball valves allow some selectivity if you want to isolate water in one barrel or another - but for the most part they remain open to work as a unified system.
Lastly the brass male hose adapter at the end of the pipe, connects the hose and runs down to a an inline shut off at the end. This allows the user to access the water anytime without having to go upstairs to turn anything on.
Step 9: Venting
I modified a 2" threaded PVC adapter and a double layer of vinyl screen.
Cut 2 4X4" square pieces of screen then crisscross the pattern. Then lightly grind down a piece of 2" pipe about 3/4" from the end (width of the grinding wheel) then cut off that 3/4" piece at the end to become the hollow plug which holds the screen into place - Cut off the excess screen with a razor knife, and thread it into a 2-1/2" hole cut into the top of the barrel.
Step 10: Overflow Components
I used a 1" spade bit for the hole - then used a 1" hose barb / 3/4" threaded adapter to make the barrel connection. I also threaded the PVC elbow on inside the barrel to act as a nut.
Step 11: Overflow
Note the 1" overflow hose which should be able to handle the capacity as it runs into the remaining downspout that used to handle this gutter that now feeds the system.