[Ironangel]

You want a slip yoke behind the 400TH and keep the two piece shaft! I just installed a 400TH in my big block truck and the only change I had to make was shortening the front shaft 1" and having the drive shaft shop weld a 1350 yoke to the end to match the 1350 slip yoke. I bought the clam shell carrier bearing and let him install it when he assembled and balanced the whole two shaft assembly. I beat the hell out of that shaft assembly with a 439ci BB that lives at 7000 rpm. A two piece shaft is stronger and will turn more rpm than a single shaft. The only time you want a fixed yoke on the trans is because there is a slip joint on the other end of that shaft. The two piece shaft assembly behind an automatic trans should have a slip yoke at the trans output shaft and a solid shaft through the carrier bearing. A bolt on Yoke with splines will bolt to the end of that (primary) shaft. The final or (secondary) shaft may or may not be a solid shaft. It depends on what carrier bearing was used, there were two types. One is a clam shell design that hangs on a stem, the other is a pillow block design that is U shaped and bolts on each side. Which carrier bearing does your truck have? The clam shell carrier bearings are designed to flex fore and aft as the rear axle articulates up and down. The pillow block carrier bearings had splined slip joints on the second yoke. That allowed the second shaft to move with the axle and slip on the splines without stressing the carrier bearing. DO NOT use a billet carrier bearing, or a bearing mounted in a rigid polyurethane bearing mount unless you use a slip jointed secondary shaft with it! Pinion bearings and transmission thrust bearings will not live without slip yokes! With your drop and going from a manual to an automatic trans, your going to need to keep the crank and trans centerline level and as close to inline with the center of the carrier bearing as possible! That may require shimming the trans tail housing up or down to keep that primary shaft flat and inline with the carrier bearing. You need to avoid any angle where the slip yoke meets the primary shaft! Your pinion should be parallel or as close as possible to parallel with the primary shaft. That way the final two u-joints will cancel each other and your world will be in harmony and balance... Similar to the image below, disregard the second u-joint...