There are two types of drive shafts that are legal. The original was white in color and has internal retainer-clips on the universal joints. The flanges that bolt to the front and rear were thinner. This shaft is widely used but the u-joint are NOT rated for the RPM's of a legend Car. The second and current shaft has 1200 series u-joint and is rated closer to the RPM's of a Legend Car. The newer gray shaft has external clips. The white shaft will weigh about 2-2.5 lbs less and should be in the 14lb range. The gray shaft closer to 16 to 16.5 range. Both are currently legal and have the same outer tube diameter.
If the bolt pattern is not the same, then it is likely you have the wrong ends on it.
Steve