999 is a heavy duty version of the 904.
A 727 will be about 1 - 1 1/2 tenths slower than a 904/988/999 if the trans and converter are built to the same extent. The difference is due to the internal rotating and overall weight.
A 2.71 low gear is available in '80's & later 904-type transmissions, but they had lock-up converters. How much it helps depends on the rest of your combo. Ideally, you can put the 2.71 low gear into an older trans because the lock-up style trans. and converters are not good for racing.