350 and 400 run the same converter which is a good thing. A 350 wastes much less power but is a little weaker in built form. 350's have a weak sprag that is not part of a 400. A 400 has two bands. Kinda weird (4L80E has two as well). 400's vary in strength stock depending on era. The pre-75? 400's have all Torrington bearings whereas the newer ones have nylon or plastic thrust washers. Gear splits favor the 350 although the 350 and 400 are very close (2.52/1.52/1.0 vs. 2.48/1.48/1.0). The difference here is that is much easier and cheaper to ratio swap with aftermarket planets and suns a 400 to more favorable ratio splits for drag racing because the clutch/apply portion of the trans is somewhat separated from the gear ratio split portion....in other words changing out hard parts to change gear ratios is easier.