Manual Steering ratio, recommendations? 16 or 24 to 1?

I'm picking all the reproduction boxes out there are either 24 or 16, where would I easily and cheaply find a 20-1?

Nowhere. It's not a cheap thing to do. Everybody else pays to play. Steering boxes are not cheap to modify or build. You could get a book and learn it yourself. That would cut a lot of the cost down.