700-R4, 4L60, 4L65E, 4L70E, and 4L80E

700-R4
While this is one of the most popular of the overdrive automatics, the 700-R4 does have a reputation for having a difficult-to-adjust throttle valve cable. As improper adjustment can cause erratic shifting and even lead to complete failure, Gear Star has developed the Fail Safe Maximum Pressure Valve Body System, which reverts to high pressure in the event of a misadjusted or damaged throttle pressure cable. A vacuum lockup system is also used that denies lockup to the torque converter under aggressive throttle application.

4L60
The latest version of the 700-R4, the 4L60 features a more sophisticated valvebody, but is otherwise the same (including gear ratios).

4L60E
A more refined version of the 700-R4/4L60, the E-series transmissions use an electronically controlled valve body. The TV cable is eliminated, but a standalone computer is required.

4L65E
There are two models of the 4L65E transmission; the early version bolts to the LT series of blocks, and the later version uses a longer input shaft and is used with LS engines.

4L70E
Found behind the LS7 engine in the new GTO, TrailBlazer SS, and SSRs, this newer version of the 4L65E uses larger front and rear five-pinion planetaries.

4L80E
The big brute of automotive automatics, this transmission is larger than the 700-R4/4L60 series and is capable of handling well over 1,000 horsepower when properly modified.

As you can see, there are many, many choices when it comes to GM automatic transmissions and the modifications that can be made to them. Of course if you're still having trouble deciding, give Zack a call and be prepared to provide as much information as you can, including the approximate weight of the vehicle, horsepower, rear gear ratio, rear tire diameter, and its intended use.

You can look forward to lots of trouble-free miles in your custom classic truck with the right transmission and the appropriate modifications for the application, and you won't have to shift for yourself.