If I understand the situation, why not try stacking gaskets? Three 3/16 gaskets gets you where you need to be for the tubes, right? Glue them together and see if they will work.

If need be, you could get a local machine shop to make you a steel spacer if the gaskets get you the height but won't fly on their own. I did that on a 440 I had that I wanted to use the original valve covers on. But the big lift cam put the rockers into the valve cover baffles. A spacer and two gaskets got me the clearance that I needed.