tyrellracing,
It sounds like you have already covered a lot of the ground I am going over and really know your stuff so if you don't mind, can you give me some feedback?
I am building 65 Mustang powered by Coyote motor.   I have settled on a Griggs front end and the first parts should arrive any day.
For the rear end,  I have been looking very hard at Duane Carlings K/A setup.  My biggest concern is one you pointed out,  the lack of back spacing due to the leading and trailing arms.  I have done a slight home brewed minitub and moved the inner wheel well edges inward about 2 inches, giving me room for up to a 295 tire.  I am concerned that I won't be able to take advantage of this added width with the K/A setup.  Duane claims that there is more backspacing than stock rear end but looking at the pictures,  I think I will run into issues.  Do you see any workarounds?
Griggs is adamant that his solid rear setup will outperform an IRS and he may be right but I would give up a slight edge to get the ride comfort.  The car is not going to see the track except maybe an occasional fun day.  And in researching the subject,  it seems that the K/A design overcomes many of Bruce Griggs objections to IRS.
What say 'ye?