SUPER, JayJay! Thank you so much for these diagrams. They will save me re-inventing the wheel and making lots of mistakes. I see that according to your drawings, the mounting board goes into the narrow space at the very back of the top bows. I thought it went into the first pair of bows from the rear with an 8" distance between them. I stand corrected. Just in time, too! For the forward dome light over the driver's seat in this custom installation, I will need to make the board bigger. The drawing of the light's outline will be very beneficial when making both boards.
Thanks again to JayJay and Bob for their invaluable input. I have been fretting for months about how to solve this problem, given the ever-shrinking availability of repo parts I need. This was one part NO vendor seemed to offer. Maybe there isn't a great demand for Murray Town Sedan dome light mounting blocks? But when you need one, you NEED one! I need TWO!!!
Marshall