Chevy did make a 4 speed truck transmission that would fit jam up in your '64. It's a 3 speed with 4th gear being overdrive (and it's been talked about quite a few times in the past on the bolt).

It's called a New Process A833 transmission and the GM RPO code is "MY6". Not the best ratios, probably not as strong as the Super T10 or the heavier duty 4 speed Muncies, but a pretty good box.

The .73 overdrive makes for nice cruising on the interstates. I have one in my '66 shortbed stepside and it was a bolt in replacement for the old column shift 3 speed.

-ftyler