ct67_72

sorry it took me a little bit.
Obviously you know that either the 400 or 4l80 are both pretty stout units.
With the 400 we usually update the valve body, and if it was something heavy or heavy towing I would probably use some borg warner or raybestos high energy clutches. the biggest problem I have with them honestly is having a good converter in it. Make sure it is a good quality heavy duty converter.
As far as the 4l80 which I think would be my choice. We do the same thing mechanically and update some other common problems. One of the largest is the AFL valve. This is the actuator feed limit valve in the valve body. There is a hole in the valve body seporator plate that needs to be drilled to .035" for oil supply and this updated valve is used to ensure the solenoids dont loose fluid pressure.
This is very common on the 4l80e. if it looses feed limit pressure it will fall out of 4th gear on the highway.