For the front bumpers, elastomeric or otherwise, IIRC they were preassembled on the brackets and then mounted to the car. Front valance would be off for this procedure, allowing 2 people to slide the bumper up and back into position.
The brackets for the front elastomerics have unique part numbers, but differ only slightly from the regular ones. Some other members may chime in here, but as I recall it's either slightly longer slots on the bracket-to-frame mounting area to allow better adjustment. The regular, chrome bumper brackets can be used successfully as the differences aren't that great. Rear bumper brackets are all the same, elastomeric or chrome.
As for the convertible top, the best instructions I've seen were published by Steve Dulcich in Mopar Muscle many years back when he replaced the top on his own '70 GC convertible. I don't believe that there's anything terribly special about the E-body tops relative to other convertibles and I've had good luck with competent upholstery shops putting on the replacements. About 8-10 hours of labour...
