Like your idea of opening up the the exhaust manifold to 2.5" that will make it breath. Big pipe out the back . .
Here is an Offenhauser 4 bbl intake, and Holley Electric Choke 390 CFM for the top side. That will give it the Fuel / Air Juice that it needs. Nice part this Offenhauser will bolt right up to your Slant 6 Exhaust Manifold, just like factory.
Got to use that 4 bbl adapter plate to get the Square Bore Holley to bolt up to the Offenhauser, for the bolt patterns.
Have had this intake manifold around for 30 plus years and have not used it yet. Got to give it up for the greater good and get it to someone who is building a nice performance project. Would like to see it get put to good use.
225 High compression, decent performance cam, great port flow and port velocity.
Sounds like a winner to me.
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Icing on the cake for RRR's 225 Slant 6 Performance Build . . .