Jon G - I don't have one to measure, but do have some other supporting documentation with diagrams that may prove helpful for your project and fellow Bolters interested in the 1955.2-59 truck radio accessory kit. I have an original 1957 Chevy Car and Truck Accessory Installation Resource Manual which provides "almost" everything needed for the accessory radio kit installation. The only thing not included in the manual is the "template" for marking the speaker installation location for mounting holes and original speaker cut out. Apparently that was only included in the radio accessory kit itself.
Anyhow, below are pictures of all the related installation instructions and drawings involved with the radio in the 1955.2-59 Chevy trucks. The page immediately following the cover picture are for the inside rearview mirror accessory install which ties into the ceiling mounted speaker. The remaining pages are the full installation instructions for the entire radio kit. Hope this might provide a little more light on the subject.
