Ok. I think I am ready to lay down some design goals.
First, it will be AD&D 1st edition, as authentic feeling as I can make it.
1) PC's will use Descending AC and Thac0 on the character sheet.
These numbers in the character sheet will be auto-transformed under the hood into Ascending AC and BTH so the value can be passed to the existing success detection code, but as far as the player is concerned, they will have their old school system.
2) NPC Sheet will have boxes for AC and and To hit value with a checkbox next to it to toggle whether the values mean Descending and Thac0 or Ascending and BTH.
Default will be toggled off. If it is toggled off, it will use the values as is, treating it as Ascending and BTH and simply pass it to the existing success detection code. If it is toggled on, it will treat the data as Desc and Thac0 and do the math to convert it under the hood and then pass it to the success detection code. This will be able to be set individually for each pc sheet.
This allows for both using the C&C Beastiary entries as is, without the need for any conversion, as well as, ease of DM entering data manually from old or OSR modules without the need for conversion.
3) PC's will utilize the 5 save system based on class and level
If I can accomplish those three things, we'll be most of the way there, I think.