It can be done, but there are numerous e body convertible only parts that are worth damn near their weight iin gold. If he has a completely rotted donor to scavenge from it might be feasible. Trying to piece the stuff together one part at a time would be nuts.
with enough time and $$$ almost anything can be done.
if you have a rusty donor body the job is not that difficult. there will be a lot of careful cutting and welding involved doing this job.
i have helped do this on a 70 challenger convert about 30 years ago, doing this was actually easier than welding in new panels for rust repairs.
here are the big differences between the 2 body styles. there are probably a few more that i forgot.
obviously the windshield header panel is different than a HT.
as posted earlier there is a thick brace that runs the entire length of the rocker panel on converts. on a HT this brace is about 1 1/2' long under the rear 1/4 glass. the rear glass guide bolts to this brace.
the inner 1/4 bracing part where the top bolts on is different.
obviously the area under the rear glass is different.
also there is a brace and drip channel that runs the entire width of the body under the above panel to catch water. on rusty cars this is usually rusted out also.
and there is a patch over a cut out area on the front of the inner wheelhouse so the top can fold into the body. on HT cars there is the mating flange joint of the outer and inner wheelhouse meet.
there is also a brace between the wheelhouses on converts that is not present on HT cars. the top pump bolts in under this brace.
as posted earlier. unless you have a rusty convert donor car i wouldn't even consider attempting this job, and i am a I-Car certified autobody / frame/ painter with over 30 years experience. i have helped restore at least 100 E-Body cars over the years.
That was exactly my point. No problems if done correctly.
The guy who had the kits was Jim and he lived in Kouts, Indiana. I think he finally sold off all the associated parts and they are coming back out. 
