If you keep the 273, the car would benefit greatly if you swap in a non-overdrive gearset with a 3.09 first gear. That's what my 65 Barracuda Commando 273 came with. (Also 64-65 slant six four speeds.) In the mid-70s, I made the mistake of swapping in a later 4 speed to get rid of the old ball and trunnion front u-joint. This was after I had replaced the 273 with a 340. Even with a 340, a 2.66 low gear and 3.23 gears turned it into a real dog off the line. I put up with that for a couple of years (at the time, I did a lot of interstate driving, so didn't want to change rear gears), then found out I could put my original gear set in the later trans.
Don't know that a 408 would need the lower low gear.
