While it is unfortunate that the studs on the ​RAYBESTOS MC36406, 15/16" master cylinder may not fit the mounting holes in your manual brake system, there are a few options you can consider.

One option is to find replacement studs that are compatible with your mounting holes. It would be beneficial to consult with automotive forums and communities, such as the ​For A Bodies Only Mopar Forum or the ​For B Bodies Only Classic Mopar Forum, where users discuss their experiences with manual brake conversions and may provide recommendations for suitable replacement studs.

Another option is to explore aftermarket adapter kits specifically designed for retrofitting a manual master cylinder to a power-brake vehicle. These kits often come with the necessary hardware, including studs or bolts, to ensure a proper fit. Researching and reaching out to reputable brake system manufacturers or suppliers may provide you with additional insights and options for finding compatible studs or conversion kits.

Please note that it is crucial to ensure the compatibility of any replacement studs or adapter kits you consider by verifying the specifications and dimensions of your manual brake master cylinder and the mounting holes in your vehicle's firewall.

Remember to exercise caution and consult with experts or professionals in the field to ensure the appropriate installation and compatibility of components for your specific vehicle and brake system needs.