The other thing is that the front sump pan (the Holden unit) is the GTO piece. The OBDII system isn't as much a drawback as you might think. LS1 Edit can be used to reprogram the OEM ECM as need be. What you can't do, a local shop usually can. The 5.3L will use the same peramiters as an LS1 or LS6 (or any other Gen III). This will allow for the same aftermarket parts as any other Gen III engine.

The 4.2L has uniqueness going for it. Not many take the road less followed. This engine has 275 HP as delivered, and with common speed goodies, 300+ HP should be easy. As for the OBDII, I'd bet there is software to modify as needed.