Hi Brian,
Fortunately you have some interesting options if you are willing to hunt for and change your rear end. You could use some of the 4 speed transmissions from the 1960s and later. You could use a 3 speed fully synchronized transmission popular from the mid-1960s forward...but any of these is going to have a 1 to 1 final gear ratio so it won't be a benefit at higher speeds. In order to get a reduced final ratio you could do one of four things: use a rear end with a different gear ratio, use an overdrive three speed transmission, use a four speed converted to use the same overdrive unit that was used in the 1960s and later or you can use a more modern transmission (like the T5) which if you get the right one will bolt up to your bell housing...like I did.
I have a 3.07 rear end and a T5 with the gear ratio chain that begins with 4.03 and ends with .86. So the first three speeds feel exactly like the "stock" AD pickup. Meaning I can put 1000 pounds of "stuff" (maybe like railroad ties or loose gravel or a cow) in my truck and drive around without any problem...no burning of the clutch, etc. Then when I shift into 4th, I get a 25% overdrive type benefit because my rear end is 25% less than the 4.11 rear. If I'm carrying a lot of weight, I can just stay in 4th. And when I shift into 5th I gain another 14% benefit and at 60 mph my engine is only spinning at around 2100 rpm.
Those are a few options. Others may come along with more.
