You are doing all this head work and talking about cams for response etc...but you want to run a hx35 truck turbo and the cast manifold?
why?
To the OP.
While I love doing things different, there is a reason why many people say stay away from building the RB20,
Hell even Roy who loves his RB20 to bits and i'm sure he sleeps curled up on the cam covers he hasnt pulled his out and built it. The reason being is that for the max 300rwkws that a 20 head will make the standard rods and slugs are perfectly fine.
If you are concerned about the engine being a little tired than just throw some new rings and bearings in it, a good oil pump and be done.
Total cost if you cant do it yourself would be 1500 bucks, and will live for 5+ years if you look after it.