You can use your same crossmember, but you'll need to cut off the mounting pad & weld it on the other side. 400's have a larger output shaft so you'll need a 400 yoke. Get a good 3 1/2" driveshaft built with Spicer u-joints. You can use the stock shifter if you modify the cable bracket to attach to the pan rail. just remember that OD will be 3rd, D will be 2nd. ect. on the shifter. If you are not going with a fully manual valve body/trans brake, you will need to run a vacuum hose to the modulator valve & wire in a wot switch to the tranny for kickdown. Just food for thought. Why not a lighter, less power-robbing TH 350? Also it will literally bolt-in with a 3 speed G-body crossmember. So will the much overlooked Powerglide.