JMHO, as much as I love the old 6s, if you are not going for an original restoration I'd go with the 400...with a good used 700R4 and a new driveshaft. To begin with I'd leave the original axle alone until you've had enough miles on it to be sure it's necessary to change it out. By the time you get the tranny, bracketry, drive shaft, 6 to 12 volt conversion, etc. you probably will not be saving much if any $$$, but you will have a more useable vehicle.