Mild LA 318 Cam Recommendations from Those Who Know

Are you wanting a smooth idle , a little cam sound? Being in Colorado is tough so a smaller exhaust like you mentioned will be good and some headwork will allow you some useable upper rpm. Dual plane intake , I would just pick the smallest cam that fits needs , if you feel the need to go bigger I would add a converter like a tight 2600-2800 for a cam that uses the phrase largest with stock converter simply because of the thin air.
There are many styles of split pattern 260-268 ish (or smaller)cams that would work well , pick one according to your needs.