This is a copy/paste from chevytalk:
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Note that this is for 59-64 only. If you have a 58 the shaft will be shorter.
Lengths of FRONT driveshafts
4-speed & 3-speed w/OD = 24 5/8"
3-speed & 59-61 Turboglide = 29 3/4"
Powerglide = 26 13/16"
Measurement is tip to tip length with front shaft completely disassembled.... front U-joint and slip yoke removed, center U-joint flange and center bearing removed.
To be on the safe side, remove your old shaft and measure it before cutting to see where (or if) you get the 26-13/16" dimension.
Updated 03/23/15
58 only info
Passenger car (exc O/D and P.G) 25 15/32"
(with O/D) 20 5/16"
(with P.G) 22 17/32"
58-64 Rear shaft 36 1/2"