Looking more closely at the Grey Elf compiled document for using Chainmail combat in D&D, it's pretty interesting. As classes gain levels, not only do they count as more "men" for the mass combat tables, but they are given additional attacks on the man-to-man tables. They can't engage in fantasy combat until they've gained a few levels (Fighters from 3rd level, but other classes from 5th or 7th), but since most monsters will still have a Mass Combat and Man-to-Man rating, it will be possible to take them on that way. It won't be easy, though. Most of the big monsters from the Fantasy table count as their hit dice worth of light, medium, or heavy horse, so PCs and their underlings fighting as light, heavy, or armored foot (in most dungeon situations) would have a lot of difficulty racking up enough 6s to take them out.
I think I will keep Fantasy Combat as an all or nothing roll. You defeat the creature or not on that one roll. But it's risky, because that monster also gets one roll to see if it defeats you. Mass combat and man-to-man combat rolls will either deal hits (HD worth of hits needed to kill) or maybe I'll keep hit points and have damage dealt.
I will probably simplify the man-to-man tables, and make numbers only for No Armor, Light Armor, Medium Armor, and Heavy Armor. I may also simplify the weapon lists, as I will definitely include more weapons than the thirteen listed in Chainmail, especially if I retain hit points.
One interesting thing I noted was that the range modifiers in Classic D&D of +1/0/-1 for short/medium/long range seem to be derived from Chainmail's modifiers. The man-to-man chart has that spread for many of the ranged weapons vs many of the armor types. But there are some where the bonus or penalty is +/-2, or even +/-3! That's a huge bonus on a 2d6 roll, much more significant than on a d20 roll.
Finally, I think I will be going through the Dungeon! board game cards to get numbers for man-to-man combat against various monsters. Those could be listed in the stat blocks for monsters, one number for each of Cleric, Fighter, Magic-User, and Thief (similar to the Elf/Hero/Superhero/Wizard numbers on the Dungeon! cards). Oh, and I got my son the newest version of the game for Christmas last year, and it's got those classes (well, being 5E based, they say Rogue instead of Thief), and I think some different monsters than the original.
Monster stat blocks will probably look something like this if I stick to the weapon type vs armor type:
GoblinHD: 1-1Mass CombatAttack: Light FootDefend: Light FootMan-to-ManAttack: Spear or Short BowDefend: Light ArmorFantasy Combat: NAMove: 90(30)Save As: Normal ManMorale: 6(8)XP: 5
If I go with something more like Dungeon! numbers for man-to-man, it would look something like this:
GoblinHD: 1-1Mass CombatAttack: Light FootDefend: Light FootMan-to-ManAttack: Spear or Short BowDefend: C 7, F 5, M 8, T 6Fantasy Combat: NAMove: 90(30)Save As: Normal ManMorale: 6(8)XP: 5