Totally understandable regarding Alto. I've done a lot of business with them, notably at the Rockland county store. Back in the 90's they had some repair staff that I would have trusted with this particular job. But I can't vouch for the repair staff now.
Just an outside the box sort of thought given the nature of this project. A J/P/J bass HAS been offered on the market before. Fender offered at least 2 different versions of the Stu Hamm Urge bass which had this pickup configuration. IIRC, one version was medium scale, and another was a 34" scale length. Have you checked one of these instruments out? It might be worth checking one out before going down the custom build path.
Just an outside the box sort of thought given the nature of this project. A J/P/J bass HAS been offered on the market before. Fender offered at least 2 different versions of the Stu Hamm Urge bass which had this pickup configuration. IIRC, one version was medium scale, and another was a 34" scale length. Have you checked one of these instruments out? It might be worth checking one out before going down the custom build path.