Like Max said, did you put the pushrods back in the proper locations? The intake and exhaust push rods are different lengths, and may be your issue. Because your van has the distributor engine, it may not be a "zero lash" design. Also, in order to set the static timing, you need to ground the EST circuit to disable the ECM from adjusting the timing until you get the timing set. Check your underhood emissions label for the location of the EST ground terminal wire, as the location differs from year to year. Failure to ground this terminal will make initial timing impossible to set, and throw off the entire advance curve.