There is a 6400 in my neighborhood that has a 350 and the original 4-speed. Ugliest most hacked-up swap I've ever seen.
There is a 4400 I see at local shows that is a 350 (actually a little bigger stroker motor) with the original 4-speed. It was done nicely and seems a very usuable truck.
If your level of experience is low, I'd suggest staying with an inline six. Some of the problems associated with a V-8 swap just go away completely, and unless you change the rear-end and upgrade the brakes the potential additional power isn't going to be of any use.
Spanky suggested a 261 Chevy or 302 GMC. I'd include the GMC 270 also, because its easier to find than a 302, externally identical, and for any practical purpose probably as powerful.
The advantage of these motors is that they fix your existing bellhousing, and with only minor modifications will also fit the front motor mount. The other issues, wiring and throttle linkage and such will be easy.
The next choice would be the more recent (relatively) Chevy sixes like the 250 or 292. A little more complicated to install, and require a different bellhousing and mounts both front and reat. However, you don't have problems with the steering like you do with a V-8.
V-8 swap can be done keeping the 4-speed, but you'll be changing lots of stuff. Its been done and I might do it on one of mine, but its not a beginner project...
