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Click on image for the lowdown. 
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8 members (Peggy M, DaveV, 2-Ton, TUTS 59, JW51, walter74, moparguy, 1 invisible),
159
guests, and
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robot. | | Key: Admin,
Global Mod,
Mod | | | Forums66 Topics128,081 Posts1,055,955 Members48,430 | | Most Online14,466
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!
Lots of work on Brad's part ... pure enjoyment (and references) for you!
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For helping keep this great
old truck web site
on the Internet for over 30 years! and not clogged down
with Ads!  Again, thanks!
| | | | Joined: Dec 2013 Posts: 41 'Bolter | | 'Bolter Joined: Dec 2013 Posts: 41 | How difficult is it to convert the 3 spd column to a 4 spd SM420? Is it simply a matter of swapping the tranny hump access or is the floor and firewall completely different? | | | | | Joined: Nov 2004 Posts: 1,552 Shop Shark | | Shop Shark Joined: Nov 2004 Posts: 1,552 | The primary & secondary transmission humps for the high hump are the same and the same is also true for the low hump, but both the primary & secondary covers must be changed, since the primary & secondary covers are not interchangeable. The automatic & 3-speed transmissions were low hump and the 4-speed transmission had a high hump.
Last edited by 4-5-6 Chevy Trucks; 02/17/2014 2:34 AM.
| | | | | Joined: Dec 2013 Posts: 41 'Bolter | | 'Bolter Joined: Dec 2013 Posts: 41 | Thanks, I just found Jolly's how-to on the GMC6066 site. | | |
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