6-71 small block build.

I would go with a 3 inch belt. I run a 1/2 inch pitch belt but I have been told the 8mm's are better and more choices. As far as pulleys go, I would go with 2 different sizes and start out under driving it and then once I had things dialed in switch them out and overdrive it a little. By that I mean plus or minus 10%. For everyday driving I underdrive mine to 6 lbs of boost and then when I want to have some real fun I crank it up to 10 lbs of boost. The idea of starting with a different block sounds good to me. If everything goes as planned you can always sell the old engine to recoup some of your money. I will state right now that once you have that blower on there and running right you will never take it off.

Jack