Re: interchangeable parts between a th400 & 4l80e
I don't know the exact list but I do know about 80% is interchangeable. They both share the same first 3 gear ratios so the planetaries are probably all the same. I don't know how the 4L80E engages the OD but I do know with a bunch of conversions that it's possible to make it a 6 speed by splitting the gears with the OD. Good for towing because it will keep the engine in the powerband easier. Instead of 1, 2, 3, 4(OD) you get 1, 1OD, 2, 2OD, 3, 3OD, 4(OD). Not really a 6 speed tranny because it uses the weak OD gear to get the extra gears.
Nothing about the valve body is interchangeable. Only the gears etc are the same. I'm pretty sure you can use the heavier duty clutch hub with 34 element sprag for the TH400 in the 4L80E.
Trying to use parts from the 4L80E to make a TH400 stronger is stepping backwards. The TH400 is actually stronger when using a few better aftermarket parts such as the 34 element sprag and clutch hub etc.