trans leaking again after rebuild

^^ sounds about right

Putting it together in 2.5 is also good, If the dang thing is clean,the sub-assemblies are all set up, All bushings and seal are in, VB is rebuilt, Pump is assembled and centered, etc.
Just slapping it together doesn't take long. It's all the setting up and fitting, and most especially, the VB. A guy can spend a lot of time in there.
And if cleaning the outside by hand, a guy can spend an hour or more right there. And the outside doesn't need to be especially clean, but if it's not, and just one tiny piece of grit finds it's way into the VB, well, you could be doing it all over again. Plus it leaves a nagging doubt in the customers mind about your workmanship. So if I'm building one for a customer, I will rather spend an hour and a half getting it really clean, just to remove that doubt. Sure on my own trannys, I've gotten away with....um....stuff.