Got my Magnum-F fully installed and now I need to go get the driveshaft shortened a bit. I already bought the 31 spline yoke (Spicer 2-3-6041X) but I was wondering, will I need to find some kind of special adapter u-joint first or will it take a normal u-joint that a local driveshaft guy will have no trouble getting?

 

My magnum f had a yoke w a 1350 u joint. You can get a conversion joint. Are you using a stock rear? I have a 9inch so I had a new 3.5 inch aluminum shaft made w all 1350 joints. The conversion joints are readily available. You'll need to give all the parts to the drive shaft builder so it can be balanced as a unit. The conversion joint will be the weaker of the two joints (if you use a 1350 up front) so you may want it at the rear so the car doesn't do any pole vaulting if a joint lets go.

the driveshaft shop should have access to all that stuff. It's all pretty common.

 

Your yoke is made to use 1330 u-joints.
Common passenger car u-joints used: 3R (internal clip), 1310, 1330, 1350 (external clip.)
Google the sizes, measure yours, and you'll know if you (don't) have 1330 joints and need a front conversion joint.