|The dash cap is a good product and I've had one on my car for over four years now, without any ill effects. The cap is secured to the original dash with a silicone sealer / adhesive, so the installation is pretty permanent. Another JH owner wrote an article on his installation, which appears in one of the back issues of the JHPS magazine. My own experiences were quite similar.
Once installed in the car, the dash cap completely covers the visible regions of the original upper dash. This includes the skirt under the windshield, the top and sides, and the area facing the cockpit. IIRC, there's also a brief lip that slips over the lower rim of the original dash, meaning the area where the switch panel and the above-glove-box trim mount.
The ball vents remain attached to the original dash but the dash cap fits properly around them. You may have some trouble getting the two long screws in the outer cupolas of the instrument pod to line up with their holes in the original dash -- but this may also be true for a stock dash.
The dash cap manufacturer's recommended method of dealing with the defroster vents IMHO gives hideously ugly results. It is far better, and better-looking, to make the necessary cutouts in the cap a bit smaller than the manufacturer suggests, and then attach the vents diirectly to the dash cap, which in turn requires cutting away part of the original dash for clearance. Similarly, rather than cutting out a hole to render visible the car's VIN tag, one is better served by attaching the tag to the dash cap with a couple of pop rivets.
In summary, if your original dash is badly cracked or cratered, the Delta dash cap is a very good choice to renovate the car's appearance. (Usual disclaimers.)