I would imagine the manifold is used so it may have been milled to fit go on a eng with milled heads or maybe your heads are milled so go to home ripoff and get a cheap propane torch take the tip off and put about 18 in of vacuum line on let the eng idle at operating temp turn the valve on (don't light it LOL) move the hose around the and see if the idle goes up or down that's the best way I have found to find a vacuum leak.....ted