Try going on RockAuto and entering your make/year and then select the biggest vehicle on the list. Go through the pictures and info to see if yours shows up. You can also try looking at model years that share the same chassis, etc. GM is famous for using the same brake system components for years across their different models. I just went there and selected a 1968 P30 and they have a firewall mounted booster displayed. You could also try looking at GMC too.
SimS
