Unless the frame on the 75 is junk, you will be farther ahead to just install what you want on that truck.
Shocks and springs will determine the ride quality. The 93 would simply have softer springs for a better ride, but those parts are also likely wore out as well
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The truck 4 speed, the manual steering, and manual brakes are just parts you can update to get what you want. The 75 probably has the same rear axle as a 93 would have, most of this stuff would need to be rebuilt or replaced on a 93 as well. Add in the condition of the motor and it sounds like your looking at a drivetrain swap. That 5.7 you have sitting sounds like a great place to start.
I've built a few trucks over the years, a 93 heater box is not going to bolt in where a 75 heater box was, if your looking to add AC, you might want to look into an aftermarket heat/AC system, they are pricy, but complete units solve a lot of problems, including duct work and air outlets. Finding a new heater core for an AC Dodge heater box may be a challenge by itself, the aftermarket AC system might be cheaper!
The door window cranks and inner window lift structures changed somewhere in the early 80s, so the old parts and the newer parts will require some work to get them to work, but if electric windows are the desire, and you want factory stuff, you will need to find power window stuff for the doors you have, or install aftermarket stuff. Look into the power door locks at the same time, you can get aftermarket power window and power door lock kits that have everything you would need. If you don't buy the cheapest crap you can find, some of those aftermarket power accessories are pretty good stuff. (Don't buy auto-loc stuff!)
If your doors need to be patch, buy the patch panels, the odds are any door from that era truck you will find needs the same areas patched.
Donor trucks for the 73-93 trucks are not plentiful or cheap, the older they are, the worse they are in both price and condition. Unless your pretty lucky, you can probably buy the parts you need cheaper then finding a donor truck with the parts you want or need. Donor Dodge trucks tend to sell for the value of the remaining parts, deals are pretty rare.
