I will be swapping out a four lug 6 cylinder axle for a 5 lug
eight cylinder axle. will I have to change the drive shaft or can I just
change the universal joint to make it work. Any help is greatly appreciated.

 

Its a 65 convertible. Forgot to add that.

 

My memory tells me the 6 cylinder is a lighter duty shaft. If it is rusty it could be weakened even more. I don't remember about the length, but all you have to do is measure. Have you searched online yet for the lengths of the two shafts?

 

9 inch rear differentials have a different driveshaft than 8 inch if I remember correctly. We needed to get ours shortened and balanced because we switched to a 9 inch. The u-joint mount juts out farther because of the larger gearing. I don't know if you are going to do this swap, but I figured I would throw it out there.

 

If this site is right, the 200 and 289 used the same shaft except the Hi-Po. Seems like your u-joint plan should work.