building a long distance touring car

Awesome idea! Some day when I retire I want to do the same.

If you are wanting fuel economy then the smaller V8 is better. Go 318 with a 2bbl.

Remember they are just big air compressors. The larger the holes the more fuel and air you need to work. The smaller engine the smaller need to feed.

Figure out how much power you actually need to move that car and the weight load. Run AC or pull a trailer. Not sure what if any. Are you alone or 800lbs of people?
Even a little steel crank 273 Ci engine with a small 4bbl will do well if not all HP-ed out.
A good dual exhaust and good tires for rolling not drag racing. Gearing around 3-1 dependent on tire height.
Ok- off soap box. Lol!