This is a good option if your budget will allow it.
https://www.gearvendors.com/cg2wdt.html
GM 4-Speed Manual Transmission Trucks
The Muncie 465 is a great transmission from a durability standpoint, but it is the widest ratio 4-speed of the Big Three. It has a granny low and so then just 3 normally usable ratios. See Final Drive Ratios and notice the extreme benefit there is to gear-splitting by using the GEAR VENDORS. After the installation – it’s just a whole different truck and you would not be able to stand driving the truck without the new gears.
GM Trucks auxiliary transmissions
You will have a 2-speed thumb switch on your shift lever to control the GEAR VENDORS (turning it on/off) while depressing your clutch. The .78 to 1 overdrive ration will increase your vehicle speed by 28.6%.
Fuel savings are guaranteed to be over 20% in both diesel and gas applications and are typically improved to right at 28%. The UNDER/OVERDRIVE™ will pay for itself while giving you tremendous performance gains in torque multiplication and horsepower.
You will climb hills, cruise, tow, do everything so much better with the Gear Vendors installed. A true 7 speed application. There is just no good reason to drive a Muncie Sm465 equipped vehicle without a Gear Vendors.