I used a mid 70's booster and a 70 C10's master along with a home made mounting bracket and kept it all under the floor too. I have a little more room to work with it being a 1 1/2 tonner, but I think there are smaller diameter boosters that would fit a pick-up just as well. All of the pedal and linkage parts are already there, so I thought that would be the best way to go. I wouldn't mount one to the firewall with out beefing it up somehow, it's just a single layer of sheet metal,
Scott
