64 GMC stepside

[Harleymike]

Hey guys. New too all this.
I just picked up a 64 from Alberta. No salt there.
Its in good shape.
I have searched this forum, but Im trying to figure something out.
Mine has the I6 with 4 speed.
These 4 speed trannies, I have heard good and bad.
Pretty much first gear is useless but very durable trannies.
But, how much HP can they take.
I plan on putting a V8 in there and would be around 400 HP.
Is this tranny capable of handling that, or should I look at a different tranny.

Thanks for any help
Mike

[Clyde65]

I would go with a 5 speed mated to the 400hp v8, for pleasure of driving and highway use.

[Captainfab]

With the SM420, SM435 and SM465 four speed transmissions, 1st gear is not intended to be used in everyday driving. You start out in 2nd gear, so you basically have a 3 speed trans. First gear is used when low speed crawling or pulling power is needed. I see no reason a SM420 won't hold up behind a 400hp V8

For a frequent driver I have to agree that a 5 speed manual or a 4 speed auto OD trans makes more sense.

[Brad54]

Captain is right about the durability of the trans.
I went through all this decisioneering for my Suburban. Like your GMC, I had a Dana 44 in mine, which gives a lot more options.
I made a chart that showed the gear splits with the Granny 4spd (yours), and a 4spd OD manual trans (since I had one and it was a bolt-in).
I figured effective ratios with the trans gears and different rear end gears, with an eye to final drive ratio at highway speeds.

With the Granny 4-speed, you could get 2.90 or 3.08 or 3.23 gears in the rear and still leave the line, with the 4spd OD I have (.72:1 final drive), I went with 3.42 gears.

I like jack-rabbit starting and 3.42 gears are still enough to pull a light trailer, so that was why I went with my choice.

5-speeds are a great choice too, but I didn't have one, and I don't know what's required to put them in our trucks as far as bellhousings and new cross members. The 4spd OD was a bolt-in for my '62 torsion-bar/X-frame truck with the bellhousing engine mounts.

-Brad