Here's what I did to two different /6 cars....first one was a 67 Dart. First thing was a 3.91 rear, then a straight pipe (first car to have a straight in 75 and have been running cars like that ever since). Nothing much was done to the engine except for tuning with a larger get to the Holley, advanced the timing, open element air cleaner, flex fan and remove weight. A standard transmission is a huge plus too with any 6 banger. The second car was pretty much the same only I removed 420 lbs from it! Both cars responded better than expected to the mods...