I did a w58 to v160 swap which cost me around $5500 including used triple plate OS Giken clutch.
My opinion, do a swap to r154 if your goal is up to 800hp and keep it auto above 800hp.
I did repair my original jdm harness twice since I went through two different ECUs. Finally I realized that if I bought a new harness I will save time and money.
For your case I suggest not to take a 98 harness since you you have aristo engine..else you need to replace some sensors.
I got 415 whp @ 17psi (pump gas) on dynojet but with the following setup
stock jdm engine
6766 .81 turbo
044 fuel pump
NA MKIV
V160 gearbox
looking for your comments on that..