2004R & 700R4 Overdrive

While neither of these were offered in 1064-1972 Chevelles, they are popular swaps for those looking for an automatic transmission with a highway-friendly overdrive.

There are several aftermarket companies that rebuild and strengthen both of these transmissions that will satisfy the horsepower your typical Chevelle is making.  Basically it comes down to what the gear ratios are in each and how much of a drop you'll experience between gears.   The table below should be of some help.

Transmission1st gear2nd gear% Drop3rd gear% DropO/D% Drop
2004R2.741.5743%1.0036%0.6733%
700R43.061.6347%1.0039%0.7030%

Which one is right for you?  No real way to know just on the surface. Many factors come into play. Engine horsepower & torque range, weight of your Chevelle, rear end gear ratio, overall rear tire size, and - most important - what will your car be used for?  A high winding small block in a light car might be better off with the 2004R because there's a smaller percentage of drop off between gears.  A heavy Chevelle with a big block might be better off with the 700R4 with its lower first gear to get the car moving.

I suggest you contact one of the numerous aftermarket transmission rebuilders of these two and tell them what you have and how you plan to drive your Chevelle and trust their judgment.