I have a (best guess) 1979 Motobecan Mirage. It doesn't have any cable guides by the FD, so I was thinking of sticking one on the bottom of the BB and running the cable that way.

Should I do that, or stick to running it over the BB? What is the best way to do it that way?

Nothing wrong with sticking a cable guide beneath the bottom bracket shell and routing it like a modern bike. Sheldon Brown actually mentioned routing the bare cable beneath the bottom bracket shell without a guide working fine for the FD (he also mentioned you can add a bit of cable sleeving/housing for the portion contacting the BB shell if protection is desired).

How is the rear derailleur cable run? under or over the chain stay? I would stick with that position for the BB as well.

If it is routed under it looks cleaner but I like the cable positioning and length better when it is run on top.

the plastic under the bottom bracket is easy to install,

just have to drill a hole in the BB shell for the screw.

You may want a drain hole in the BB shell also , let out condensation in the frame.

There were clamp on over the BB guides back in the day, but that is a real challenge to find , know of a bike shop that has been in business since the 40's?

This is what I would use to make that decision. If the chainstay braze-on is on top, run it above....on bottom, run it below.

thanks for all the advice. RD cable runs over the BB and the braze-on is on top of the chainstay, so i'm gonna run it over the top. i was just worried that was the wrong way, since other stuff on the bike had been set up wrong. but seems like thats a fine way to do it, so i'm gonna stick with that. thanks again.