My main concern is the single point U bolt mount on round bar uprights will allow the mill frame to twist relative to the bar - that's going to create all sorts of headache.
I'd be looking to use either SHS for the vertical and weld in a piece of Angle to make a V so the SHS sits sung in the V at all times.
Something like this.
The same could be done with a half piece of round pipe welded onto the frame that the bar could snug into.
I'm not 100% positive it can't be don't but when I laid it out I could only have about 20" of usable bar. I don't want to be too close to the nose of the bar.
If it worries you a cover could always be welded for it.
And I have a felling dog in the chassis of the saw that's in the way of the bar clamp
Standard CS Alaskan style mills are unbalanced enough as it is so leaving the dogs on just unbalances the setup even further.
Then the bar clamps are about an inch wide. So even if I could use all 28" of the bar I'm already down to 26" with both bar clamps. Then another inch for the falling dog and 2-3" for the sprocket. I could get very close. To 22". But the. Again when will I ever need lumber that's 22" wide. Aside from a slab for a tabletop or something of that nature. And then you get to the point where my ms460 will struggle milling something of that size. I had wanted to make it adjustable but the work involved to get another inch or two didn't seem worth it to me. Unless I'm missing something which is very possible as I've never milled before.
You will always run across wood that is just a touch bigger than your bar width.
The 460 should have no trouble in 20-22" wood. Wide boards are easily ripped in half or 1/3rd etc with a hand held circular.
An even better design is to not use bar clamps at all and drill the bar through the nose and mount the mill on bar holes - the clamps are after all a bit of a kludge, they get in the way and reduce the cut width and if they are not tightened then the chain is toast.