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Apr 26th, 2026 | | AD Chevy Trucks  Over 6,000 pictures Brad Allen has an awesome collection of Chevrolet factory pictures that he has set up from film strips. Check them out!
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| | | | Joined: Oct 2003 Posts: 5,152 Cruising in the Passing Lane | | Cruising in the Passing Lane Joined: Oct 2003 Posts: 5,152 | Anything that has ever been installed might be found in a junk yard. GM did not make an overdrive for the SM420.
The two options that come to mind are the Ranger and the Gear Vendors. The Ranger goes in front of the transmission and the Gear Vendors goes behind.
The Ranger is much less expensive than the Gear Vendors though neither is cheap. I think you'd be more likely to find either used on craigslist than in a junk yard.
Other options to consider are changing the rear axle to one that is available with more highway speed friendly gears or swapping to a transmission with an overdrive.
1955 1st GMC Suburban | 1954 GMC 250 trailer puller project | 1954 GMC 250 Hydra-Matic | 1954 Chevy 3100 . 1947 Chevy COE | and more...
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