I'm speaking from personal experience. Back in the mid 70's I had a '69 Swinger 340 car that I put a Holley 800 spread bore on and it ran the best of 13:22. Later I installed a six pack set up and the Dart instantly ran a best of 12:88 at 114 mph. The only thing done to the engine was a cheap 500 lift, 300 duration hyd. cam from JC Whitney and full length headers. I had a B&M 3500 Super Holeshot convertor and Zoom 513 gears.