Tramp,
New Process 833 overdrive is the way I recommend you go. It's the easiest, most reliable way to get overdrive in a '64 pickup. It has 3 forward gears with fourth being overdrive. First gear is a little lower than your factory 3 speed, second gear about the same, and third straight drive (1:1 ratio). Fourth gear is overdrive with a .73:1 ratio.
It's the same length as the factory 3 speed (so no driveshaft modifications needed) and, if you can get the original shifter it will require no fiddlng around with the handle to get it to accomodate your bench seat. Plus there are no solenoids you have to worry about wiring up or going bad on you every few years. Only problem you might have is finding a heavy duty bellhousing with a 5 1/4 inch retainer bearing opening because the input bearing housing diameter is 1/2 inch larger than your stock bellhousing. A bellhousing from a truck with an SM465 works perfectly (and they're relatively easy to find in Georgia).
I have one of these transmissions in my 1966 shortbed stepside 1/2 ton truck and it's an enthusiasts' dream come true! My truck has the same rear end gears as yours (3.73) and was turning about 3000 rpms at 60 mph. 65 was my absolute maximum cruising rpms on the interstate, and that was
killing my 230. Plus it's not safe to drive on north Georgia's interstates unless your vehicle is able to cruise at 75 mph. At the time that was my
top speed.
But now with the overdrive, I've gone from turning 3000 rpms at 60 mph (3200 at 65), to 2600 rpms at
80 mph! Talk about pushing people out of the left hand lane! At the same time I also installed a 292 I built with a mild cam, headers and 4 barrel. There is no lack of low end torque or high speed cruising ability now.
Remember this trans is called the "New Process A833" (also sometimes referred to as GM RPO code MY-6). There are at least two versions of this transmission. One casting has a half-Mopar/half-GM design. You want to avoid that one because it will require the original bellhousing (which is aluminum and doesn't have rear mounting cast into it). The other type is the standard GM 4 ear casting that will working with an old-style large truck, cast-iron bellhousing. Find that one and everything else is an easy bolt-in fit.
-ftyler
