Any of the kits from Jeg's, Art Morrison, Chassis Engineering etc. are all pretty good. You need to have some tube notching skills as well as be a pretty good welder to do a nice job. You can use a chop saw to notch tubing at pretty much any angle you want with great results. 90% of the tubes I notch in my shop are done with a chop saw including high quality chrome moly tubes that are TIG welded. You make one cut through the tube at a 45 degree angle, and then rotate the tube 180 degrees and cut that side. You will have a notched tube that will need a little grinding to fit. 90 degree notches will have equal cuts on each side of the tube, and angle notches will have one cut bigger than the other. I custom bend all the cages I install as it is easier for me to do it that way and I can get a better fit.
S & W makes the best fitting cage for a Mustang, IMO. all you have to do is notch the ends and it pretty much "just fits", at least that's been my experience. I used a Jegs, Summit, and Morrison cage in the other 3 cars and the S & W fit like it was made to. Only other cage that fit better was one I have in my Maverick...a 14 pt cage that was made by a place that is no longer in business.
I just got an S&W kit for the Falcon - all the tubes are pre-notched, with the exception of the "pro street" tubes I ordered extra that go through the package tray. Looks nice, complete - and came FedEx for a total of $98 shipping. Nice folks too, shipped the kit in like two days after order, took me completely by surprise.