Use thin paper gaskets two per side they are made thin for this reason. Don;t use standard gaskets they are to thick.
The reason aluminum intakes don't seal with the metal valley pane is due to expansion of the aluminum.
We just got a new aluminum Dart block for a build. The sleeves were .001 above the deck cold. At 200 degrees right out of the oven the sleeve was under the deck. The sleeves had to be removed and the block decked the sleeves have to be .003 above the deck or the fire ring won't seal when hot. That is how much the aluminum grows. and th block lifts the head off of the fire ring
I have seen aluminum Hemi's hold the valves open when cold after adjusted when hot. Valve's need to be loose for warm up and readjusted when hot. That is how much heat plays a roll with aluminum parts.
Use two thin paper gaskets per side made for sealing aluminum intakes. I usually would glue them to the valley pan top and bottom on each side. The glue is only used on the pan sides so they will not stick to the heads or intake for re-use. also all new intakes need to be fitted to the motor . Deck height and head thickness or intake angles will affect sealing.
