I've never had a slant six nearly as fast as Charrlie_S's, but my 67 Barracuda used to have a slant six. Stock except for a four barrel carb, Offy intake, stock exhaust manifold with 2.25 inch pipe, automatic and 3.23 gears, it ran low 18's in the quarter. Milled the head a hundred thousandths and swapped in a 264/.440 Comp Cam (220 @ .050) - that dropped a full second off the quarter mile ET. Could have used more torque converter as my 60 foot time was only 2.50 (same as with the stock cam and CR), but I never bothered changing it or gears or anything else (until I swapped in a 360). Compared to the stock cam, the Comp Cam really woke it up from about 25 mph on up. It made for a nice daily driver, in town or on the interstate (could easily cruise at 80-85), with good passing power and a decent top end (110-115).