If your engine is a 1980 20R, the peak torque is a low 2400 RPMs, with peak HP at 4800.
If the transmission is an L43 4 speed, 4th gear is 1:1.
Typically the choice of gearing is based on where your peak torque is and what RPMs your want it to be in the RPM range for the speed of your pickup.
In other words, with 4.11 R&P and 33” tires, in 4th gear your RPMs will be 2511 at 60 MPH. At 75 MPH your RPM will be 3139 in 4th gear.
In my 1986 XtraCab, 22RE with a W56 5 speed, 30” tires, 4.56 R&P, at 80 MPH my RPM is about 3200 in 5th gear, which is just about in the middle of my peak torque curve and a comfortable cruise RPM.
Gnarls.
