Danny,
There are a number of folks that believe that by having a 4 speed transmission that 4th gear is an overdrive. Even if he has a passenger car 4 speed - Muncie, Saginaw, or Warner, these were all 1 to 1 in 4th gear just as a standard 3 speed or your original truck grannie gear 4 speed is 1 to 1. That means that whatever your final drive ratio is in the top gear, that is as good as it gets. An overdrive will increase speed and reduce RPM's as it is less than 1 to 1 - generally 30% less. Example: 3.90 rear end ratio less 30% = 2.73 final drive ratio in overdrive.
It is entirely possible your friend has something like lms suggested but it certainly is not a stock application. Post a picture of it and someone can tell you what it is. Fred

1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine