Have you confirmed that the gasket surfaces, both on the cylinder head and the manifolds, are flat and straight? Eliminating the heat riser valve was a BIG mistake- - - -it needs to be there and functioning properly. It's a pretty simple procedure to line ream the manifold holes a little oversize and fabricate a shaft that fits, which will last 3 days past forever- - - -especially if you make it out of stainless steel.
I'd suggest getting both manifolds milled flat, and leaving the bolts that hold them together slightly loose until both manifolds are snugged up tight against the head. Also, be sure the alignment rings between the head and the intake are in place, and aren't too thick- - - -they don't seal vacuum, but they do keep the manifold aligned so the gasket can seal. With a milled intake manifold, you'll need to grind a little off the rings to keep then from holding the manifold away from the head. Last suggestion- - - -buy and install a Remflex gasket- - - -it's about twice as thick as the original equipoment type, and does a really good job of sealing. Good luck!
Jerry
Edit: DO NOT bolt the ends of the exhaust manifold down. Those dowel pins in the head and oversize holes in the manifold are there for heat expansion. Bolting the ends of the manifold down tight will make it stress-crack eventually.
