drive shaft??

i am new here and i have a ?????? i have a 71 and i just changed from drum to disc and i am planning on putting in a 406 with a th400 i have the crossmember but dont know how to measure for the drive shaft can someone tell me thanks tom

You want to measure from center to center of the U-joint. With the yoke in the trans and the yoke bolted to the rear (without the U-joint straps installed), measure the distance from the rear of the trans yoke (where the U-joint strap goes) to the same spot on the rear yoke. Make sure the vehicle is sitting with all of it's weight on the suspension. If the suspension is hanging, you will end up with a driveshaft that is too long. Also make sure the yoke is not pushed all the way into the trans, you need to leave some room for travel (usually you want to leave about 1" for travel). If you are getting a driveshaft made, I got mine from a guy in Michigan, aluminum shaft made from 1/8" wall, 3.5" seamless tubing, custom length, balanced, including U-joints and shipping for $225...best deal I found anywhere. If you want his name, PM me and I will dig through my receipts.