Just tapping some 1/8" wall steel will not really give you enough threads (number of threads in the material) for an 'appropriate' size (diameter) of threaded item for this situation IMNSHO.
You usually want at LEAST three full threads in something, five threads is a better minimum to aim for. Which usually works out to be about the diameter of the thread that you are using.
ex: 1/2-13 threads You want five full threads, that means you need a thickness of (5 threads) / (13 threads/inch ) = .385 inch thick material needed. Close to the 1/2 inch thread diameter.
3/8-16 threads : (5 threads) / (16 threads/inch) = .3125 inch thick material needed. Again, pretty close to the 3/8 inch thread diameter (just 1/16 less
)
Same with metric threads, just different math (because of the way the threads are called out).
M10 x 1.5 : (5 threads) x (1.5 mm / thread) = 7.5 mm material thickness needed. Again close to the original 10mm thread diameter.
You have an 8 ft long bar/countertop. Figure that sometime SOMEONE is going to get up on that counter, could be more than one person. Could be you standing up on it to change a light bulb or swat a fly, could be several folks being rowdy, could be the bar from Coyote Ugly.
So figure that the counter/bar has to be able to support several hundred pounds SAFELY. 8 ft long top could rather easily fit 4 adults on it, let's do a SWAG of 4 people x 250 lb/person = 1000 lbs up on top of the bar. Plus the weight of the bar itself.
Use enough strength in your feet and attachment method to safely and easily support that load. It doesn't have to fly or race, so make it PlentyStrong.
You can weld a nut to your frame to get the threads, you can weld a plate of the appropriate thickness to the frame and drill and tap threads into that, you can (maybe) put a rivnut in, you can weld a threaded bung in, etc, more than one way to Git'r Done.
Make it PlentyStrong though.
Restaurant or other 'commercial' type of space, I'd probably go for MINIMUM 1/2" or M12 threads for a threaded adjustable foot pad (thread with a foot pad on the end). Heck, IIRC my washing machines or clothes dryer usually have about 1/2" diameter adjustable threaded feet on them for leveling. You want at least that strong or better IMNSHO.
