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I think in a pretty stock application you wouldn't see much disadvantage to running an adapter plate. The adapter plate acts as a carb spacer which actually helps, so if you lost anything in the adapter plenum it would probably be a trade-off with the extra height. I don't think it would be enough to notice one way or the another.
I'm not a fan of the adapter plate mostly because it just complicates things for the most part. There are more gaskets required, and you may end up modifying or having to replace the brackets for your throttle and trans cables because the carb will be sitting higher. Is the intake that you could get a good deal on a square bore?If it was me I would save up to swap the heads, cam, and intake all at once. You could spend $30 or more to gasket a new intake on, and then have to take it off and get new gaskets again when you swap the heads. Also, if you really want to make some more power with that motor, and end up finding a good deal on some vortec heads, you would want the vortec intake to go along with it.