Hmm, a 289 in a Cortina should be very interesting.!!!! The parallel 4 bars work great, another option to consider would be a triangulated 4 bar. Your plan for the parallel 4 bar would work fine!

For the street, the coilovers work best if angled in from the bottom to the top at an angle between 25 to 28 degrees. Some manufacturer's suggest their coilovers mount at 30 degrees.... Mounting the coilovers vertically leads to a harsh ride and is best left to the drag cars... If it were my choice, I'd go with the new crossmember and the coilovers angled in.

A panhard bar is the easiest solution to control lateral movement. The mounting height on the rear end housing can also be raised and moved forward so that the panhard bar is forward of the coilovers and above the housing. Also, the nearer you can have the panhard bar to parallel with the ground at ride height, the better off you are.

I would say you are on track, if you have any other questions, ask away!!!