That would be a record to do a quality V8 swap in a weekend.
The 4.0L was new for the 1990 model Ford Ranger
I mentioned that you already have the M5OD-R1 and that the transmission would bolt right up to a 4.0L V6, eliminating the need to hunt down a seperate transmission.
In previous threads you wanted to turbo your 2.9L and perform a V8 swap. You said your dad wouldn't help you and you would be doing this yourself. If I remember right, you were grasping at straws when it came to wiring and carb adjustment.
So, we've ruled out engine swaps. Thats fine. I think you will see the biggest performance gain (and MPG) if you ran through the truck and gave it a COMPLETE tune-up. Go through everything. Synthetic fluids in your trans, rear, engine. Exhaust will give you 1-2hp (thats it, not joking either) but it will give it a nice sound. Putting a cone on the end of your intake will only take away that 1-2hp you got from the exhaust. Put your hand by the factory airbox when the engine is warm. You will feel a lot of warm air being blown toward it. A better idea would be a high flow replacement filter. You will still draw cool air in and the engine can breathe easier, not to mention cleaner.
And one last note that has to do with both performance and the M5OD-R1. Your current M5OD-R1 has a low first gear of 3.72:1, second is 2.2:1, and third is 1.5:1. If the new transmission that you are swapping in lived behind a 4.0L the first three gears would be 3.4:1, 2.05:1, and 1.31:1. The taller gears might have a negative impact on off-the-line acceleration.