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cam is different Spec difference is 250hp (L-30) had a WCFB or 4GC carb,hydraulic lifters and 2" exhaust. 300hp (L-74) had an AFBaluminum carb, mechanical lifters and 2 1/2" exhaust.
the 327 was offered in four different power ratings depending on the vehicle it was in. The base models produced through 1965 with four-barrel carburetors and a 10.5-to-1 compression ratio made 250 horsepower and 350 foot pounds of torque. Another version manufactured through 1968 featured a four-barrel carburetor and the same compression ratio, but produced 300 horsepower and 360 foot pounds of torque. The “performance” model of the 327 at the time also used a four-barrel carburetor, but bumped the compression up to 11.25 to 1 to produce 340 horsepower and 344 foot pounds of torque
PowerPaks to double hump heads also when yougo from 250 to 300hp.