I have a ‘68 Cougar w/a 289. I have a pretty mild combo with stock heads and it runs nice. Comp xe 256 cam, stock heads, performer low rise intake w/ 600 Holley, Pertronix flamethrower 3 dizzy, 1 5/8” full length headers, 9.66:1 comp (if memory serves me correctly). I can’t tell you what the exact hp is, but after resetting and recurving the distributor, it feels like a rocket compared to how it left the factory. If I didn’t have the vacuum headlights, I would have went a tad bigger with the cam, but would probably not see much more power until a head change. Just remember this: a small engine needs torque and velocity as much as anything. Especially in a heavier car. I wouldn’t go much bigger than 170-180cc cc on your intake port size unless you plan on going stoker in your future. I also would go much bigger than around 224 @ .050 either for the same reasons.Too big a cam will be slower in a bigger car. 600 cfm is right where that 302 is going to be happiest. Especially the if you are in a street car with any weight. I don't see how you wouldn't be able to achieve 300 hp with the factory heads though. I mean Ford did 271 HP with the 289 with TINY valves and ports, and like someone else mentioned Shelby best that to the tune of 350 with ported. The Shelby 306 hp 289 still did this with FACTORY heads. And there was really nothing special about the factory hipo heads besides screw in studs and guide plates. Without that, there was no difference between the hipo and non hipo 289 heads as far as port volume. A mild cam within the specs that have already been suggested, good intake like an RPM or RPM air gap, headers, good ignition, and a little tuning should have you pretty dang close if not beyond that with today's camshaft designs. I would stay away from a single plane in your heavy combo for the same reasons that I wouldn't go too big with the cam or the carb. There really is such a thing as too much of a good thing with YOUR heavier car. Keep your velocity high. It will win out over port/plenum volume every day but on an engine dyno shoutout where peak numbers are touted over a good curve. Besides that, if you look at power curves, the RPM dual planes give up very little at high rpms compared to a LOT of single plane intakes, but absolutely kill them on the lower rpm to midrange and are VERY close on the upper end. Which do you think will be faster in the real world? The heads would just put you that much further up that ladder.
