Texas Backroader, what is your budget?

The 318 is a good platform to start with. Does it need rebuilding at 100,000 miles? This is where you have to ask your self, if it needs rebuilding and if it will need new pistons, the machine work is not cheap.

Many engine shops in Texas can make you a drop in crate motor that will be about the same as a rebuild but will maybe cost a bit more, but they do the work.

Getting a used Hemi or a V10 will also require you to rewire the truck with a computer control and all its sensors, fuel tank and fuel lines. You will not be able to use your 4 speed so get the trans that goes with this Hemi or V10. Your stock dash will not work with them so kobble up the dash too. You may get most of these items with a wrecked Dodge truck.
And making motor and transmission mounts to fit. Then the front suspension would need boosting for the weight of the V10.... Lots of work. But it will run real well.