On both my diesel landcruisers, I have used a 4"dump pipe, then 3" exhaust.
My thoughts being that a 4" dump acts as an expansion chamber, eliminates back pressure completely so the turbo spools freely. Even if it is an effect that works for only a few seconds, it can still make a difference in driveability.
The exhaust cools, and condenses as it travels through the system, so for me 3" seemed more than adequate. Most people tell me the 4"dump is over kill, but anecdotally, it seems to work well.
One of the things with an exhaust system that regularly gets left hanging, is eliminating as many bends as possible. Toyota sure left a bunch of low hanging ripe fruit here.
Check out my pics in my 105 project thread for an exhaust systems that could be almost identical in a diesel 80 series, largely the same for a petrol 80.
Tightest bends in the system are in the 4"dump at 75°. The rest are generally no more than 45°.
Good free unrestricted gas flow, and quiet!