I installed my directly to the firewall. The hydroboost unit is from a later '98 or so Astro. It normally has the angled firewall mounting bracket, but I swapped it out for a '94 Astro strait mount bracket. The later Asto unit input rod is almost exactly the same length as the stock master cylinder, and it has the same pedal hole size. Long story short, with this combo you don't need the threaded actuating rod.