To my understanding knocking out the bearing cups and installing an American mid bb should do the trick.Man, you can fill a thread. How many pages since the last picture of the bike? I had to dig a little bit. You've decided on a threadless stem, right? I think that should set the tone for the build. 3 piece would be the choice for design cohesion. I've personally never gone 3 piece on an old frame, there are apparently adapters that make it all work. Have you looked into that end of it ?
I'll be putting the battered profile ahead stem on there with a set of steel MX bars. On the front I'll run a v brake with the p2 fork. I'm currently looking at some nice single bmx levers to use for the front brake.