In Aus, the RB30ET had bigger brakes, disc brake rear, different gearbox, flywheel and clutch compared to the RB30E manual.
I’d hazard a guess that Holden engineered the RB30DET with better brakes and driveline and probably cooling system for a reason. Also wouldn’t surprise me is there were some chassis strengthening also done…
So why not start your upgrade process from the perspective of upgrading the body, brakes, driveline and cooling in preparation to dropping in a more powerful turbo engine?
After all, on public roads, it’s better to be able to stop quickly than simply take off quickly and find the 4 cylinders brakes are woefully inadequate. You don’t want to find yourself in the following situation after spending lots of $