I got my 289 heads out to 155cc on the intakes, 55 on the exhaust. They were pretty much maxed out on useful port volume.

These are generally smaller in stock form than D0OE 351W heads. I didn't measure mine, but I had read 289 heads start around 126cc... and I removed a LOT of cast iron out of these things! The bowls are very, very, very thin in many places. One intake bowl actually cracked, causing the engine to hydraulic. I fixed things, and epoxied the port, and it ran trouble free for several years after that.

Good Luck!
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