The hardest problem that I see is measuring it.
Also at 36" long I bet thermal expansion starts to come into play. A 25 deg F temp difference will probably move the bar out of spec.
Not That I have any experiance but I would machine both ends clean, measure, then take another cut and sneak up on the dimension. If the bar is long enough and you measuring device will reach, you coud have it stick out the back of the head and measure it in the lathe between cuts.