An adj. clutch cable and quadrant... and a firewall adjuster, are all really good items that need to be installed, IMO, to get the most out of your new clutch... not absolutely necessary, but a darn good idea!
The Spec unit sounds good to me, I have heard alot of good things about them... never tried one though. I ran the D/F setup on my old combo that went 11.70's on slicks and it held-up great!
And the steel bearing retainer just makes life so much easier... good thing to address while the tranny is out
Just reitterating what everyone has already said to stress the importance of taking care of all the weak-links found in the mustangs factory clutch setup.... if the money is there, I'd take care of all these things.