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