318 complete combo suggestions...

6-8 hr trips, I'd run a 2.94 gear ratio with a 26-27 inch tire.
Nice flowing dual exhaust. Manifolds work fine if you don't want headers
Keep the cam small. Something around a .427-.450 lift, and duration around 210-216 @ 050
Heads are the name of the game for gas mileage and snap. Have a nice valve job done, and if your motor is year correct, hardened seats or buy heads that have hardened seats and do them up. Really don't need to port, but cleaning up casting burrs and barely opening up the ring under the seat in the bowl area will pay dividends along with removing the valve cover bolt lump in the intake port.
I'd run a Weiand Action plus intake and a 600 cfm carb of your favorite flavor. Even a Holley 3310 would work great.
I'd consider a good aftermarket converter.