I've been looking over the Swords & Wizardry rules which are the OD&D rules. The original rule set only had 3 classes, Cleric, Fighter, and Magic-User, and all three classes had d6 hit dice. Just how iron-man are you wanting to be now? Also, every time a character advanced a level, he rolled his hit points all over again from scratch, using the new number of dice and any ap;plicable constitution bonus, with the only proviso being that a characters hit points could not decrease. In the event of a sucky hp roll, the character's hp would remain at the previous number.
To quote the rules --
"Character Classes
There are three character classes in this game: Cleric, Fighter, and Magic-user. Your Referee may also have invented other character classes, or may be allowing optional character classes and races from other fantasy games.
Reading Character Class Tables
Level: Refers to the level of experience of the character.
XP: This is the number of Experience Points (XP) needed to advance to this level.
BHB: This is the “Base Hit Bonus” added to the attack roll. (See Combat)
HD: This is the number of d6 Hit Dice at that level. Characters re-roll their HD each time they advance a level to obtain a number of hit points. (If you roll poorly, those hit points don’t go down, however.)"
