Need info on slant six engine

Just bought 65 barracuda with slant six and 4 speed. It has a holley carb and offenhouser intake . Did this come on slant sixs originally or did someone add this?
I have a 66 cuda sl6 4spd. I've added frt disks, bigger t-bars and an 8-1/4 rear axle, w/ a Mopar rear sway bar, and 14in Mustang painted Magnum 500 wheels. It's a kick in the a$$ to drive, wouldn't think of swapping in a v8, though I have plenty of parts to do that, if I wanted.