Perhaps it is too obvious to mention, but the AD&D DMG is invaluable. (I was checking it out just the other day ... and there are 20 mental illnesses listed. 1d20 for madness, anyone?)
am converting Master Dungeons M1:Dragora's Dungeon to use with GCC Rpg- it has intelligent apes living in a lost undeground city, sound pretty much appendix N material.
the follow up module Master Dungeons M2:Curse of the Kingspire looks ripe for conversion as well
It's pretty cool that you sussed that out. Hints of Appendix N adventures were showing up long before the DCC RPG was announced, and I was intentionally trying to go "deeper" (in this case, R. E. Howard). While the stats will take work, there is some really useful Appendix N style adventure material in both M1 and M2.
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character sheets, judge tools, and the world's fastest 0-level party creator.