Follow-up: The '68 Mustang, "1FSTPNY", has been "out-and-about" for 6 months now. After a few hundred miles of "test flights" and some slight suspension tuning I went up to an autocross being held at Shelton (WA) airport. I decided, "If I'm gonna break 'er, I'm gonna do it today". And proceeded to mash the course. Man, the results give me a rush of pride, and same should go for you! Absolutely exceeded my expectations. With the locker rear end, she grabbed the course and just tore it up. Flat, solid and steady even around the tightest turns at each end. Just a little over-steer, perfect for feeling a nice, smooth, controlled transition as traction slips. No jump, shudder or hopping, at all. I can get the rear to come around nicely and set up for the straights. The slaloms were tight, with almost zero body roll. The TCP frame connectors locked the chassis; front to rear, and the Mustang II coils overs with rack & pinion in front .... altogether a fine driving experience!
NOTE - The aluminum torque arms began getting loose in the threads, so I went with steel. Also added side plates over, and just outside, the wheel arch (belt and suspenders) just to add some stiffness in that high stress area of the side rails. Final weight distribution, with full tank of gas and driver, is 51% Front, 49% rear.

For Daze. It handles really well, altho' so far haven't really stressed it. I'm still working the "shake down cruise", or might I say "flight testing". It is totally stable, straight line under acceleration or braking. Feels tight and confident. Once I get 500 miles on her, I'll take her down to the skid pad, and maybe run an autocross or two, then report back.
Thanks again, Day !!!! Nice work!!
 

I have more pics, drawings, and can offer some tips from my experience. ONCE AGAIN .... ALL CREDIT TO DAZE!!!! YOU WILL RECOGNIZE HIS INSPIRATION FROM THE PICS HE POSTED LONG AGO! HE LED THE WAY, I FOLLOWED.

Also a little peak under the hood (I hope you don't mind). 351W stroked to a 408.

Stepping back a bit, here's the wood mock up, and the subframe prep.

Here's a view from the rear, up into the left side of the IRS.

Since I have welded in TCP Frame Connectors and stiffener, I have a convenient place to bolt in a pair of torque arms.

The spring adjustment details are shown here. At first run, I was about 2" too high, so I got shorter, stiffer springs, and now they're perfect. (IMPORTANT NOTE:, the '67 & '68 Mustang have 2" wider flange-to-flange distance on the original car, so shortening the LCA and half shafts is not necessary.)

This (not very clear) shows the rear end swinging up into place, pivoting on the forward rail bushings.

Here's where the cross member attaches to the side rails. Two sets of poly bushings and flanges, with a 5/8" Grade 8 bolt through 'em. The frame tubing is 1 1/2" x 2 1/2" by 0.188" wall.

OK. Ready set go! I have my Jag IRS on order. 3.31:1 Locker.

I see a ton of good info here. (Sorry, I'm a newbie and should "look before I post"). Great info and maybe all I need to do the conversion.  Thank you!

Tho' more data would always be welcomed! Thanks again!