I have an 8in x 8in x 1in steel plate I want to use for a project. It already has four 0.375in holes I want to plug, to make the plate "contiguous".
I'm thinking of drilling and tapping the four holes to 1/2-13 (0.4219in tap drill), then "slathering" JB Weld on bolts (or studs) spin them into the holes, sticking out a little proud on each side, then once the JB Weld cures, milling or fly-cutting down the surface of the plate.
That's the hard way... That's very doable, but it's an inch of threading with a lot of "diameters" engaged, you'll need a dead straight start and some elbow grease, and you'll want to be generally on your A game. It'd work for sure though. I'd prefer to use a locking compound over JB weld for something like that. In either case the end use must be considered.
Same threading idea, but different (and way easier). Drill from both sides and tap each side for an quarter inch NPT plug. Way less effort due to the shorter tapping distance, and WAY less risk due to a much "beefier" tap with much less engagement. (Make sure to leave it tight enough to leave a thread or two of the plugs showing after it's tight... Oil the threads of those plugs, thread one in. Yarn it right down until the square head rounds over. You're gonna mill it flat anyhow. Plenty tight. All you can give it. Turn over and repeat. Depending on how you're gonna use this- It's clearly not this, but brass plugs made flush are kinda reminiscent of an old timey braised repair. If that doesn't entertain you (and fair enough, it's a pretty visible detail), cast or steel plugs are quite stout, and "almost' go away when you make them flush.
Or depending on what you're doing, ream the holes (or drill and ream) for a heavy press fit to a standard pin, and rail a pin in there.
Or, if you have welding equipment and you're not afraid to use it... With holes that small? Don't weld in plugs, drill the whole dang thing out to a half inch so you can get in there, or if you don't need "solid", just burn some weld right in the top of each hole and dress it up.