OP here again:
Excellent discussion everybody!
To those who wonder why I don't have a welder.
I did have a nice new MillerMatic 211 but I used it so seldom, and was so out of practice each of the few times I used it, I decided to sell it after a few years. I needed the floor space more than the welder so off it went. I had a welding booth around it to contain the sparks from my wood walls, and it was just too big.
I decided to proceed with my "bolt plug" idea. Almost went with the solid plug and press fit idea, but I had the proper tap and tap drill on-hand for 9/18-18 holes. Next time I need to plug holes I'll try another approach.
The bolts are just sitting loose in the holes in the pic below. Now I need to figure out how to cut the bolts to the proper length so they sit a little proud at each end after epoxying or loc-titeing them in, then milling or fly-cutting them to the surface. I have a horizontal band saw but the bolt heads make it a little problematic as far as work holding. Tried to find 9/16-18 threaded rod at the local hardware store which would be snap to cut to length, but no stock at two stores. Didn't feel like buying on-line then paying shipping and waiting.
Thinking about gluing them in, then milling the heads off, etc. Or slitting sawing them off.
The object of the exercise is to make an adjustable mini-pallett as the pic below.