This is my OD&D project. The basic idea is that the deeper one goes into the adventure, the further you travel along the OD&D publishing timeline. Encounters in the wilderness surrounding the dungeon will use the original Chainmail rules. Exploration of the ruins above the dungeon (an ancient, crumbling castle on a hill, of course) will use the 1974 boxed set. The first dungeon level will use Supplement I: Greyhawk. The next level will use Blackmoor, and the third level will use Eldritch Wizardry. I don't think I'll need more than three levels, but one could add the Arduin Grimoire material for the 4th level and the later Arduin trilogy books for levels 5 and 6.
Adding monsters and magic items the deeper one goes ought to be a cinch. The real tricks here are that the rules change. Blackmoor has hit location tables, for example. And Eldritch Wizardry has an inexplicable alternate initiative system. And the players will find that their characters change, too. The strange either/or multiclass system of OD&D gives way to the more recognizable format still largely used today. Wizards will find their spell lists very short initially. Thieves won't even exist until level one of the dungeon! It will be an odd play experience, to say the least.
"Okay, you descend the stairs into the dungeon. Here are your new character sheets."
"Hey, my hobbit fighting man is now a halfing thief!"
1 hour ago