There are countless options
Just one: crank position sensor wire has been damaged
Valid reasons also include wrong phase of moon, or retrograde venus, accounting for engine age and zero input data given by you
Do not do it
It will have harsh 1-2/2-3 upshift and downshift and a few other things you won't like
ls/gs400 (vvti) and ls/sc/gs430 (5 speed) transmissions have different valve bodies and different control parameters
It's ok to swap valve body from your old transmission to new one however
But...
He has compatible ecus that can be purchased from him. There are a few ways to tell how much the ecu is supported, i.e. if ecu can be serviced in USA, or has to be shipped my way, or can't be modified using my technology
The new ECU drives ok, abs/trc/vsc ok, no engine or a/t trouble codes BUT a/c and AFS not ok. Need time to fix it. Must be related to it being first ever SC430 6 speed ecu that I modified
Valve body wise, plan to pick it in a week.