318 rpm drops to 400 in gear WHY???

I've got a similar issue with the 318 in my Duster. I've recently rebuilt the 318 to below specs..

*302* casting heads
Weiand Dual Plane High Rise Intake
Comp Cam 224@050
Stock rebuilt bottom end
Hooker Headers
Holley 600 Vac Sec (NEW)

Having the same issue where it idles fine and then as soon as I put into Drive it stalls.. I have to re-look at the timing and vacuum levels which I havent done yet and also check for vacuum leaks as well... Damn frustrating...