Not much of an update, but better than nothing.
I've started the rebuild. Engine has been stripped down and I've added some ARP mains studs. Block has now been taken to the machinist for some work which I should have back in the next couple of weeks.
I've ordered a new 390ci rotating assembly from Fraser @ AES Engines which consists of a Callies Compstar crank, Callies Compstar rods and CP pistons. The setup is designed with boost in mind, so it should be sweet.
I've found that my Tex Z-grip is due for a rebuild as it is down to wearing on the rivets. It lasted me 2.5 years and about 70,000k's of abuse, so I think it did well. I will have it rebuilt and put it back in. It might be a bit of a test for it holding a boosted 390ci setup, but I'll worry about that if it turns out to be a problem.