Ok, here is a third conflicting answer.
You didn't say what kind of transmission you have in there now.
If you have a 3 on the tree, a 3 speed + overdrive will be almost a bolt-in. These were a factory option on Chevy cars and trucks from 1955 until about 1969 and are still fairly easy to find. They will bolt to your stock bellhousing.
If you have a 4 speed, the easiest swap will be a T-5 from an S-10. That's what I installed on my truck about a dozen years ago and I have been happy with it ever since. This will also bolt to your stock bellhousing. If you use the truck seriously hard, you might want a stronger 5 speed like an NV4500. This one will NOT bolt to your bellhousing without an adapter like this:
http://www.advanceadapters.com/produ...ter-plate-kit/
If you have an automatic, you probably have an adapter on the engine now that will let you use a Chevy pattern transmission. The most durable of these (in stock form) is the 4L80E, but that one requires an electronic controller. Non-electronic transmissions which will work are 700-R4 and 200-4R. These are not as strong in stock form as the 4L80, but can be modified to survive severe usage.
Ray