Grapevine is tough for the grade and altitude. Any naturally aspirated engine will be singing up there. The 7.3L Ford diesel will be a little better, but it is very much lower power with close to same torque as the V10. Fuel usage will be lower/better. :) Power and speed wise I think will be tough to show significant improvement over the V10 under $20k.
Would you consider a full sized van? 2011+ GM 1-ton passenger van with 6.0l/6-spd should be well under $20k with under 100k miles. That 6-spd is really helps keep engine in powerband.
An Example
