which belt is ideal for a 4lb lower and 2.8 upper ?

 

67" (gator back# 4080670) is what I use on my 2\4 combo.
that's with a aux. idler kit.
Before that I used a 66 1\2" (#4080665)

 

My stock belt fit

 

I've squeezed a 67" NAPA belt on with a new style Metco 100mm Idler Setup on it, but it was a little tight for my liking. I got a Gates 68" belt, that that put the tensioner almost exactly in the middle of it's tension range.