I have to start with one or the other. Should I go with TCP's full subframe connector setup first, or do the engine bay with monte carlo bar, export brace, and maybe a front sway?
Are the costs roughly the same?? If so look at a less expensive set of subframe connectors.
Personally I think you should do all of what you have suggested.
Well I plan to do all of these eventually, but money dictates that it be piece-by-piece. The engine bay work would be cheaper, but if I'm going to do one or the other first, which will make more of a difference?
To me, the export brace is needed worse. Your shock towers have probably shifted some, hopefully your frame rails have not! I view the export brace as being more necessary to keep the front chassis straight. My export brace, with welded-in reinforcement, and 1" front sway bar made a huge difference.
i agree with the others, export brace and monte carlo bar first. then go with the subframe connectors. once you have these items on, THEN you can go for suspension mods. the stiffer you get the chassis, the more effective your suspension mods will be. most people who get the 620 springs, and then say they ride nice, either have no subframe connectors, or are hard core racers.
Look at the spacing of the bolts, the Shelby ones are evenly spaced, Mustang ones are not, they are in two groups of two. If at all possible add the re-enforcing plate when you do this. It must be welded in but adds strength to the mounting point for the bar.