In my opinion the 300 straight-six is the best engine for a 4x4. It has loads of torque from idle to 3,000 rpm. With proper care they can last 300,000+ miles. From what I have seen they are durable (my buddy dropped a nut in his carburetor with it running. to this day we still haven't found it.) My dad also has a 1994 2wd 300 with 130,000 miles and there is no difference between now and 1997 when he got it. Pulls good, even for an automatic. He's trailered a few vehicles with it and it did great. When it was a daily driver it would get 25mpg on the interstate.
If you already have that engine it would be best to stick with it. It is a good one and there are a lot of hidden costs on swaps, especially the fuel injected ones
Can't really help you with what will bolt to the transmission.
Sorry, wasnt trying to write a book lol.