I remember that in 1969 we thought that there might never be a faster Mustang than the Boss 429. Of course nowadays even a brand new V6 accelerates as fast as the fabled big block (385 series) Boss 429. One must remember that pre-1972 gross horsepower figures for American cars are pretty much useless. I would guess that the 375 gross horsepower in the Boss 429 equates to maybe 275-290 in modern net horsepower. The gross torque ratings were usually closer to reality. Possibly close to 390 lb/ft in modern terms. These are guesstimates of course. As for top speed, with only a four speed manual transmission, there was always a trade off between acceleration and maximum speed. 127 or 128 mph is actually fairly generous, must have been with a taller than average rear end axle ratio, with at most four speeds the old cars were usually limited by their redline.