What if anything is stopping a guy from using a manual steering box from a vehicle other than the "standard" Vega, Corvair, or modified boxes?
Nothing is stopping you. There many steering boxes out there besides Corvair, Vega, etc.... in a bucket Seen a few VW van boxes, a farm combine box, and others. Just get an old box and look at it and see if it might work and how you can mount it. You also have to consider how easy it is to get replacement parts.