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Old 04-29-2013, 11:16 PM
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66 GMC 4 speed bolt pattern

I'm looking at buying a 66 GMC that has a 305 v6 in it and a 4 speed. My question is will this tranny bolt up to a 350 without any problems or is the bell housing different?

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Old 04-30-2013, 11:53 AM
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So I dug through the archives and asked my boss who has been doing this since he was about 7yrs old.

Back in the 60s all the SM420 4speed parts literature says "except the V6"

Which means the V6 equipped SM420 has a custom-length maindrive (input), input bearing retainer, bellhousing etc. I do not know if the bolt circle on the heavy truck V6 is the same as a standard SB/BB Chevy. I DO know that this is considered a 60* V6 - Think 2.8L, 3.1L etc (opposed to the 90* V6s like the 4.3L). I bet if you noodle around for SM420 parts, you'll get some info.

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Old 04-30-2013, 03:02 PM
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So it most likely wouldn't work without some work? Also would a 4 speed from a 69 chevy 3/4 ton swap in its place pretty easy?
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Old 05-02-2013, 10:03 AM
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I don't know a lot about the V6, as most Canadian trucks had the inline 6 or, occasionally, a V8.

The guys on the 60-66 forum would probably know.

The other thing is that truck 4-speeds usually had an extremely low 1st gear, often referred to as "bull low" ... and many owners just skipped it altogether, starting out in 2nd gear ... which was more like 1st gear on a 3-speed.

So ... if you want a 4-spd, I'd suggest you get a V8 and 4-speed from a Camaro or Nova all in one chunk. that way you know you'll have the right bell-housing, flywheel, clutch, etc.
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Old 05-02-2013, 10:29 AM
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You can also ask over here.
The 1947 - Present Chevrolet & GMC Truck Message Board Network - Powered by 67-72chevytrucks.com
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Old 05-02-2013, 11:00 AM
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gear ratios.

The trans you chose depends on what you want to do with the truck, pull a trailer , drag fire wood trees out of the woods, then you would want one with a "granny " low gear ratio, for a stree-occasional drag race then a passenger car trans might be better. Chevy V 8 truck parts will all bolt right in. I had a friend put in a big heavy Clark 5 speed in his turbo boosted
chevy V 8 truck. low first gear and an overdrive, more suited for a 2 1/2 ton rated farm truck.
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