It seems that quite a few problems in developing a d20 Conan game go hand in hand with the problems I've experienced from trying to develop a d20 Lankhmar game.
It seems that such early sword-&-sorcery fare such as Conan & Leiber's Nehwon are essentially low-magic in comparison to the D&D standard. There really is no distinction between arcane & divine magic (as it is in the real world--more often than not, magic from a priest of an accepted faith is considered "holy," whereas magic from opposing faiths is considered "unholy," and often labelled sorcerers, witches, or wizards).
IMHO, the way the D&D version of Lankhmar dealt with this was to eliminate the priest classes altogether, remove the spellcasting abilities of the ranger, bard, and paladin, and rework the existing mage classes to fit the world of Nehwon--elemental mages (mainly fire & ice mages), white wizards (who used the divine spell list), and black wizards (who used the arcane spell list).
However, I wonder if d20 Modern (or d20 Ancient, in this case), may work better for these kinds of settings. Any spellcasters/psychics are treated as prestige classes, and it seems best suited to deal with a humanocentric setting such as these.
In addtion, Action Points seem to lend themselves well to "heroic" style play, the Defense bonus system can account for improved defensive abilities without relying on magic &/or armor, and the system practically encourages mutli-classing (IIRC).
It seems that this would be an easier option for such settings rather than trying to shoehorn D&D into a very different setting. Just MHO, though.