Ok I got a set of Mr. Gasket (ya I know) Underdrive pulleys, my car has the old V-belts on it. I have heard that if you went to the serp. belt set-up and put underdrive pulleys on, it will slow the engine causing the waterpump to slow down and over heat, is this true or somebody just blowing smoke??

 

Depending on the condition of the car in question; it might or not be a strong headwind of "smoke" a blow'in....
A low mileage, well taken care of high mileage 5.0 will not run into any problems, except possibly with the alt. pulley(I never use 'em; the stock alt has enough of a hard time as it is). If you have an origional stock radiator & a stock or replacement waterpump; it might run 10-20 degrees hotter, but still within acceptible range. The intelligent thing to do would be to put a real coolant temp & oil pressure gauge in the car..... Then, you'd know if you need to revert back to the old V-belt pulley setup after driving it with the "new" pulley configuration....

P.S. Eventually you get to the point where you've installed a Fluidine aluminum radiator, 3G alternator, a big electric fan, & it doesn't make a difference what pulleys you want to put on; you'll be fine no matter....