Any idea what the humidity and barometric pressures where on that day? Looking at the color of the grass I would bet money it is very humid their Which means a lot of humidity means thin air
I would focus on getting rid of the bouncing when leaving the starting line, look at the rear leaf spring angle first at rest and then jack the car up so the rear tires are off the ground and the suspension is not loaded, don't jack it up by the rear end housing and leave it on the jack in other words
If the rear spring shackles are pointed straight down or pointed slightly forward from the upper eye then see about relocating the eye forward one or two inches in it or making the upper bracket longer with the bolt holes further forward so the shackles will be pointed back toward the leaf spring eye at rest and not pointed straight down or forward under acceleration
Fixing the shackle angle will make your sixty foot times better, that should eliminate the bouncing also, if that is what is causing it
Please let us know what you find out and what you decide to do BTW, what body style,A,B or? is your car It looks like a 1970 Plymouth B body from the passenger side and looks some what like a 1967 to 1976 Dart A body from the drivers side
Nice car