How would you go about converting from BECMI/1e to v3.5?

[Voadam]

I could see how the 2e to 3e conversion document might work for 1e, but I would think the numbers would be kind of skewed if I used it for BECMI NPCs.

Would BECMI ability scores really be equal to 1e and/or 3e ability scores? I would think they'd not be as powerful. (Of course, when it comes to 2e versus 3e regarding Strength it isn't the same because of exceptional strength precentages.)

BECMI has the normal 3-18 range for ability scores, with no percentage strength, a +1 bonus at 12 and topping out at +3 for 18. You will be fine translating the scores over directly from BECMI to 3e IMO. The +1 or so variance in bonuses that is different between the systems at specific attribute levels is insiginificant for conversion purposes.

BECM and 1e are very similar to 2e. 1e has a few different class mechanics, different initiative system (using a d6 instead of a d10), and different xp awards, but I used material like modules and sourcebooks interchangeably between basic, 1e and 2e. BECM changes race even more and is a little lower power than AD&D in general (ability bonuses top out at +3 at 18)

Note that giants and dragons got significantly upgraded for hp and damage in 2e as well as in the Mentzer companion set in BECMI versus 1e and the older Moldvay basic/expert sets.

Dragons are hugely scalable in 3e though so it is easy to pick one to match your desired target CR/EL. I think scaling down to ogres is a good option for matching the originals if it fits your campaign flavor.