Mailbag: Transmission and Gearing Recommendations for a 454-Powered ’74 Impala on Big Wheels

1974 Chevy Impala 4-door big wheels

(Image/Deleon Mich Auto Sales)

Q: Recently, I purchased a 1974 Chevy Impala 4-door. It’s a pretty big car, so I decided to swap the small block 350 with a big block 454. I had a buddy build the 454, and we got a lot of parts from Summit Racing—from the Moroso 7-quart oil pan to the K&N air filter.

We figure it will make between 475 and 500 horsepower.

We’re stuck on what transmission to use. Most people have advised me to go with a TH-350 or a TH-700-R4, with a 2,400-rpm stall converter on either one.

What would you suggest?

I’d also like to change my rear end.

I’m in my early 20s and live in South Florida. Everyone my age down here runs with huge rims. I’d like to use some 24” rims myself, so I want a rear end that will work with tires that size and still perform well on the highway.

Any advice you can offer will be appreciated.

A: Nice ride!

Let’s start with the transmission. A TH-700-R4 is a good way to go because of the overdrive you can use on the highway. You can also use a lockup torque converter so there’s no slipping on the highway.

As for the 2,400-rpm stall converter—it’s hard to say without knowing the specs on your cam. It should let you take off nicely, and with a TH-700-R4 lockup unit, it won’t slip at highway speeds.

Your Impala should already have an 8½-inch, 10-bolt rear end. Keep it! That’s a strong unit. Then you can use either a 3.42 or 3.73 gear to get those big wheels rolling.

This is another in a series of weekly Q&A Mailbag sessions with Summit Racing‘s tech department, in which there are hundreds more. Click here to see them all.

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  1. Question is that can stock th700r4 or th350 handle that bigblock torque!?
    I got th400 with same power range..

  2. G. Simpson says:

    That sounds like a pretty cool project. Not sure about the build on the engine, but 475 – 500 hp is definitely possible with the 454 depending on the ingredients. I’d be more concerned about the torque. The TH 400, or the 4L80-E if you want to keep the overdrive my be a better option.

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