My pulleys are approximately 1.09 ratio.
The March High Flow pulleys are .92
The March Performance pulleys are .81
I have ran underdrive pulleys on my 460 and small blocks before with no problems.
I have .96 ratio on my 460 with no cooling problems, and my alternator charges fine at idle with two massive electric fans.
I have essentially the same fans on my 390 with the same alternator so I don't think charging would be a problem.
They are kind of pricey for what I feel the performance gain will be.
On my 69 mustang, 390, the bottom radiator hose is close to the p/s belt, so a smaller crank pulley would help there.
It's not rubbing or anything, but it's a close fit on any FE mustang.
For those who are running them, your thoughts please.