I did a TON of house rules over the years to be able to run Hyboria using D&D rules. Since not every campaign needs to be a bunch of chaotic warrior-thieves running around, here's the scoop on how to still make variant campaigns workable:
Races: Human gets 32 point ability buy. Dwarf, Elf, and gnome actually are 'mythic races' and come from Vanaheim/Asgard legend and if players really want to play them then use 29 point buy and harassed anywhere they go. If you're dying to use halflings, house rule that they're just short humans. As for half-orcs, they actually work great: rule that they are 'degenerate' humas with beastial-bloodlines. HUMANS: need to pick a race from the Conan RPG handbook.
Bard, Cleric, Druid, Sorcerer, Wizard: Can't take until 3 levels in a differen't class. Character levels 5/10/15/20 must also be in a non-spellcasting class. Sorcerer and wizard can create their known spells from all spell lists. Balance: Armor penalty applies to ALL spellcasters.
Ranger: no spells; instead bump back up to d10 hp's.
Paladin: no spells; no detect evil; instead give bonus feats at 5th and 12th level.
Fighter, Barbarian, Monk (Khitian), Rogue: No changes necessary.
Unearthed Arcana: Warrior, Expert no change; Spellcaster as others above.
Psionics: These have worked well in my campaign. I just treat them like 'spellcasters' above and require them to be of the Serpent-Man (Yuan ti) degenerative offspring bloodlines. No non-human races used from the book.
Oriental Adventures: All follow the rules of spellcasters (samurai, ninja require 2 non-levels an then follow the 5/10/15 rule too). No non-human races used.
Nyambe: As D&D class restrictions. No non-human races used.
Spells: Take any spell that you don't want players using and bump it up a level or two. Remove Raise Dead, fireball, magic missile, and a few others.
Magic items: Magical gear is still fun to hand out but I just put a limit on what PC's can own: one permanent item per 3 levels and one temporary item per 2 levels (e.g. potions, scrolls, and wands). Players just have to choose what they want to keep.
That's about it. We really like having more options in our campaigns and it's not hard to logically fit them into Hyboria.
One last thing: We helped playtest the COnan RPG and enjoyed it, but we chose the brevity of the D&D combat system. We're going to allow all of the races/classes/magic as-is (superior to the D&D system encourages players to purchase the book right?
I'm actually looking towards doing "GESTALT" characters this next campaign from UA, because I don't want to have to go to the really high levels this campaign (we went to 14th on this latest one and at that point the challenges get too D&D-like).