Idle is out of control

For a motor to "Rev" it needs "air" so either the throttle flaps aren't closing fully or it has an air leak (unlikely given you have replaced boots, vac caps etc)

That one photo looks like you "might" of bead blasted them as an assembly im i correct? That grit, if so, can get wedged in places you might not think! Also check those two phillips headed screws per butterflies. If You or someone else loosened them then butterfly might of gotten cocked in venturi and not fully closing. 6K might sound high but those butterflies don't have to be that open to achieve that rpm level. Also double check choke plunger seating position if applicable.  

Tex
Idle adjustment screw is backed out, It is screwed in just enough to stay in place.
All
Thanks for responses
I guess I will take the rack off AGAIN. Gonna double check the butterflies,
That seems at this point to be the only thing left.
I rebuilt these about 15 years and never had any problems. Guess I got lucky!!!

Comparing what mine looked like after a full re-build... to me, it "seems" that the butterflys on your carbs are not closing fully.

If you hold them up to a light can you see light around the butterfly’s ? Mine had no light showing 

 

About to take the carbs off again to check the butterflies. I have what will seem to be a very elementary question.
What is or is there an easy way to install the air boots between the carbs and the airbox?
Is it just generally a PITA?
I have been going through the airbox letting my wrists get chewed by the plastic!!

There is no easy way out of this. You take the airbox boots off when removing the carburetors? when i install the airbox boots I try to go from the outside and use a flathead screwdriver to push the lip through the hole but if one is being persistent I will shove my hand in the airbox. If you're just trying to remove the carburetors I leave the boots on and pull the carbs up and out to the right. Im sure there is a better way to do this though.

It depends on the age and condition of the boots, mine are new OEM and are flexible enough to "roll back" on themselves (I am sure as they age harden I fill need to find a new way)

Unbolt the airbox and push it back as far as possible.  See what it's hitting when you do this, and rearrange things until you can't anymore, then wrestle the carbs out.