Quoted: Anyone know of a machinist capable/willing to dimple this barrel?

Can you provide a photo of dimpling?

I have always used that term to describe adding a small detent 180 degrees from the gas port.
The purpose is for a setscrew attached gas block to lock at least 1 setscrew into the barrel so the gas block will not move forward.
I did not look up the YHM part number you have specifiec for the gas block, but I will just assume it's the new one with a rail on top and the bayo lug and sling swivel on the bottom right?
That is a clamp-on gas block, so there is no need to dimple the gas block seat.

If you don't find a barrel off-the-shelf, I specialize in custom barrels and uppers.
I can make you about anythng.

In your quoting of throat lengths attributed to the different chambers, please realize that you are quoting lengths that do not feed from a magazine.
These are lengths of loaded ammo that highpower competetors use in single shot fire from 600yd prone.
Magazine fed ammo will be loaded to about 2.25" long and there is no need for extended throats, in fact, you don't want an extended throat.
For accuracy, you certainly don't want a typical 5.56 chamber as the throat is grossly oversized.
What you want is something like a Wylde or Compass Lake 5.56.
These are really just 223 chambers run in deep to headspace more like a 5.56 with the accuracy of a 223 chamber.
More fun with chamber reamers here

As for the choice in uppers, I don't know why there is this desire for a DPMS receiver.
Go with an M4 ramped CMT.
They are nicer receivers than DPMS.
Realize that I am a DPMS dealer telling you this and I buy mostly CMT receivers for my builds.

If color matching is a big issue, then coat both the upper and lower receivers at the same time with something like Duracoat and they will match perfectly.
That might even be a good time to choose another color scheme besides basic black.

Quoting JTAC: My understanding of fluting, and I make no claims to being a structural engineer, is that removing metal is not ever going to make the barrel stiffer than the same barrel unfluted. The claim of increased stiffness I believe is when comparing barrels of equal weight, one unfluted and one fluted. The fluted barrel of equal weight starts as a heavier barrel, with more diameter and more material. Hence it is stiffer than the thinner, unfluted barrel.

This is exactly right.
Everyone seems to have this idea in their head that fluting an existing barrel makes it stiffer and more accurate.
This is NOT the case.
You have to flute a bull barrel and then compare it to a barrel of a smaller diameter, but the same weight, then the fluted barrel is more accurate, but it's because it's stiffer, not because it's fluted.
The fluting just allows you to run a stiffer barrel of the same weight.
Of course that's not a fair comparison to make because it's comparing two different barrels, not the same barrel before and after fluting.

I have my milling machine and indexing head almost setup, just need to connect it to the phase converter and I am good to go.
Here is the drawing I did for Watcher a while back with the important specs removed to protect my time investment: