I built a 389 for a customer w/ a 65 GTO recently- the original numbers matching block had been bored .090 to fit standard 400 pistons and the crank was already cut .020 under, so I ended up sourcing a replacement block and crank for this project.
The owner wanted his 389 to be as period-correct as possible, so we used the original 8-bolt timing cover- for your application the later 11-bolt timing cover would be much better (no pulley alignment problems w/ the later model p/s pump and alternator). You'll need to use a later model balancer w/ this timing cover as well. We also used later model no. 11 heads to keep the compression pump gas friendly.
Ordering forged pistons for the 389 will take at least 4-8 weeks, so plan ahead. I also put the forged 5140 replacement rods in the motor and had all of the usual quality machining processes done (blocked bored and honed w/ torque plate, had the rotating assembly balanced, decked, align honed, etc.) to ensure this motor would live a long life.
Geno