The 90 LT5 has the most to gain from porting the top end. The primary runners are 33mm compared to 34.5 (about) for the later 93s and up. Also, the primary injectors are sealed at the top of the injector instead of at the bottom. This allows you to remove ALL of the shroud on the primary side injectors -making the 90 IH
thee ideal year for top end porting.
The starting point for most top end porting is approx. 36mm. There are nuances that improve over that (but Pete n Marc each have dibbs on one of my family jewels, should I divulge a sacred FBI secret
)
As far as port matching into the heads, I believe there is merit to tapering the port all the way down to the valve guide. But, there is some real concern for "punching through" if you don't have a road map, and pulling the head(s) means pulling the motor...not a cheap or easy thing to do.
P.