My vote would be none of the above.
Trail Dusters are indeed uber rare compared to Ram Chargers but I've never bought in to that making them more valuable. I mean the differences aren't more than a few badges. It's hard to argue against an as shipped 4x4 unless it's got the 318 and ya have no use for the 4wd. I've never been a fan of laying frame or much of a drag racer so those options aren't my cup of tea.
I'd go basic big block 2wd street terror. I've got an LRE and it's a lot more fun on twisty roads than my 318 4wd RC but it's not a convertible. The G3 Hemi swap is the current big thing - to the point that Holley is actually making kits for it. It's "only" 3-5x the cost of vintage iron. Tempting but ya know, a couple 440s in the basement.
As for 4wd to 2wd, no need to cut frame rails if your parts donor is a 72-93 pick-up. Just swap the engine cross member and upper control arm mounts and then move the rest of the front suspension over. I'm pretty sure they left both sets of mounting holes (small block vs big block) in the cross member through the end of the run. Another good trick is find the much more common 2wd RC from the 80s and swap tubs. There is a difference in the frames because they added rear seat leg room when getting rid of the pop top but you can get around it with different height body mount bushings.
Thank you with you assessment " basic big block 2wd street terror.", you pretty much see it spot on. My idea of a drag truck is not a trailer troll that shows up once every couple months to lay rubber down the 1320, but a ride I can drive to the track, go on cruise meets, weekend stoplight parties and such. To me, a drag truck can be more than just a quarter mile at a time. If you removed 6 emblems/decals on the truck, it's a Ramcharger. Added bonus of an extra seat is all to to the interior and frankly, the plymouth grill is really just a dodge grill with a gaudy pot metal emblem in it.
As for cutting the frame, that was a mis-wording on my description. I have a 1979 truck for parts, so I'm cutting the front frame off of that and thinking of transferring the center mount crossmember. That is one way I'm considering on converting the front suspension. My other choice is to ship off the clip to Magnum Force and have Ron design a whole new front suspension for it. I like their work, have used lots of their parts, got two Transformer suspensions going on other cars and always loved how much better the cars drove. As for the convertible aspect of it, I will probably get another roof and doors for racing Though it would have a full cage, I would still run the roof, but a gutted one that was lighter and easier to remove and replace when needed. I have an Indy headed race engine already, so if it don't go in my CUDA, it would probably go in this truck. I greatly appreciate everyone's impute, makes it feel like an interesting build is what is needed in my life. Like I said before, I have like 5 years til my other rides are done, so I'm just going over ideas til that times comes.
Around here, TrailDusters are very rare animals. You'll see 50 Ramchargers to every TrailDuster. AAMOF, it's been quite a few years since I've seen one. 
