Nice. So did the "Titan of the Tripower" Mr Dick Boneske gave you some bad advice seeing his idea just cracked your intake?
Butler seems to sell a Mr Gasket set that states "steel" block off plate.
https://butlerperformance.com/i-2445...tegory:1234822
Felpro offers a gasket with the block off, BUT, I believe what you wanted to do is cut down something like the putty knife blade to the same shape as the block off filler shown in the photo and then insert it into the hole. This way it would be within the gasket material rather than on top of it so when you torque down on the manifold bolts, you are torquing down on the gasket as you should and not on the combination of gasket and steel which causes the bolts to continue to pull down over the additional thickness until they bottomed out on the head and cracked.
https://butlerperformance.com/i-2445...tegory:1234822
There are other ways, like filling the head with furnace cement to create a plug. Then us a gasket that has no heat crossover to begin with and create your seal. KRE has a "trim-to-fit" with no crossover passage, you would just have to use a stock gasket to match your intake passages which should not take too much trimming, or the RA IV having the crossover insert and leave it in place. Both of these would work with the furnace cement fill.
http://www.krepower.com/Pontiac%20Gaskets.htm
I read in an older hot rod magazine to just pack a bunch of aluminum foil into the passage to block it. Seems cheap enough and probably might work.
I would not go and get another intake. Have yours repaired. They make a good cast iron welding rod that should do it, or brazing might even work.
The underhood temps can be a problem. Don't use any kind of hood insulation as it traps heat. You might consider an open hood scoop insert, not necessarily the ram air tub set-up. You could have 2 inserts, one open and one closed should you encounter any rain/water. Easy enough to swap them out, but the open insert will help vent underhood heat.
The heat soak is a problem when you go to restart the car after it has been shut down hot. If it sits for any length of time, the alcohol blended fuel will evaporate over time. The cure most have found is to add an electric fuel pump to supplement the factory mechanical pump for faster start ups. You also want a return fuel line going to the gas tank if you don't already have one of these. The AC cars had these. You will need the matching gas tank sending unit having the provision for the 1/4" return line. This can help.