Dollar for Dollar you will be happier with the 5.0L, the truck will feel like a new one.
The 351 is too big in outside dimensions and a replacement V6 is almost a waste of time.
Why do all the conversion work and not get the pedal power.
Not always but some people have reported an increase in MPG with the 5.0L.
Theory is you don’t need as much pedal to get moving and less to keep it going.
Luck on your choice.