My daily driver is a 1972 dodge d100. The first year of the square body dodge you like. The body changed a bit in 80 something, but stayed the same until 93.
My truck has the 318 3 speed and about 500,000 miles. Motor was rebuilt around 10? years ago. It has done everything I asked. Even a bed FULL of 14ft 2x4's when I repitched my roof. It can and HAS done 200 mile round trips with no problem. If I had the money, I would do a 4bt or 6bt diesel swap.
I would look for a 90-93 d250 diesel. I've seen trucks with almost 1 million miles and still daily drive dependably (until it was totaled)
Something to consider, no matter what brand, is maintenance. My 40 year old truck keeps up with traffic, and is dependable because I keep up on the maintenance. You could by something that is 5 years old, and be a piece of crap because of high mileage and neglect.
My motor flat spotted a lobe on the cam. Being broke, I put in a 360 cam from a project motor, and BAM, I'm back on the road. If you're mechanically inclined, get something you're comfortable working on. My first car was a 1985 Nissan 300zx. It was an auto, and I wanted a stick, so I converted it to 5 speed. So if you can't find exactly what you want, you could always fix it up to your liking.
Good luck. I'm rooting for the dodge, but I'm biased