By the way, it's not that hard to adapt adventures to your game: i'm about to play Castle Amber, The Desert of Desolation and Tomb of Horrors in DCC RPG, it's not that hard to get what's important to keep and what you can change.
How are you converting the levels in DCC RPG with Tomb of Horrors and the other classics? Since DCC RPC only has 5 in the beta, and 10 in the upcoming rules, what level spread are you using for S1?
I want to keep that feeling that the funnel never really ends. So it's fine to pit the PCs agaisnt adventures thought for a party of 2 levels higher than theirs, at least. I forgot I think I will run something else first, like The Silver Princess module. Castle Amber seems a little too much for the PCs at level 1.
To convert adventures I simply take a look at monsters and change them a little bit. For example, in the Desert of Desolation Random Encounters table there is an entry : 1d8 bugbears. Now, bugbears are quite powerful in DCC, and I expect PCs to be at level 3 when they go or that adventure. I would probably change that entry to 1d8 hobgoblins.
In S1:tomb of horrors, I think that most of the traps are actually a challenge for the players' wits, so it really doesn't matter what level they are. I will for sure tune the monsters down: there is some 6-armed golem with over 100 HP; I think it will just have 10d8 HP, and less immunities.
For me, a weak monster is 1-2 HD, medium is 3-6 HD, Strong 7+ HD.
I don't know, if you can be more specific i can tell you how i'd convert a specific part of the adventure.