The 3.4 is a headache to work on, and has gotta cost a fortune. The 3.1 is tough as nails, cheap and easy to work on, and has great low end torque. And they run pretty much forever.
I'm sure it can be done without too much hassle. GM parts especially of this era are very interchangeable. What I would do is sign up here.
The BEST Source For All Your GM 60šV6 Needs
They should either have an existing writeup, or would be able to tell you everything you need for the swap.