In the absence of guidelines or actual rules in this area, I've decided to go with D12 as a standard beginning point for monster HD, especially for retro-fitting existing adventures to the game.
My reasoning is that:
1. Basic/Expert D&D used D8 for monster HD, which matches the highest character class HD (for the Fighter). So I've done the same with DCC monsters matching the Warrior's HD.
2. DCC characters are stronger than old-school D&D. D8 HD monsters in a standard slug-it-out fight just won't last long.
For example, in my current group there's a 1st level dwarf, with standard sword and board. On a successful hit with both sword and shield, he does a minimum of 8 HP damage, and maximum of 21 damage! (He was fortunate to roll a 16 Strength, but even without that it would still be up to 17 damage). A 2HD monster using d8 HD has an average of 9HP. No contest.
I usually go even further and just choose monster's HP, with around 8-10 HP per HD, and a standard +1 attack modifier per HD. These figures work for us - they make monsters shit-your-pants scary, but not impossible to beat. (Unless of course you take on a dragon, in which case the party is probably all dead after the first breath, so the dragon's HP and HD won't matter...)