318 rebuild for turbocharging

I can tell you what I did with my 318 and I also live high altitude at 6280 ft. (Colo. Springs)

I ran a 318 magnum for multiple reasons. It was cheap to source a used block. The roller cam was important to me in the fact that I too, wanted to run EFI. I did not want to attempt to break in a cam and experiment with EFI at the same time. Otherwise I would have to break in the cam with a carb and since my setup (megasquirt) was going to control ignition and fuel, I would also have to slave in an ignition system, it was easier to just find a roller block. I ran EQ magnum style heads because they are stronger between the valve seats (wont crack like stock), flow a little better, and the deck surface was already true. I used ARP head bolts.

I accepted a goal of 400-450 hp and it seemed right for a 318 at my altitude. I used the stock rotating assembly which has been proven to hold up as long as it is rich enough. My pistons sat pretty low and my final compression ratio was about 9:1

The thing about "needing a little extra boost on top" because we live high altitude is untrue. 10psi at sea level is the same as 10psi at 10,000 ft. Air density is all the same under compression because the turbo is the compressor, not the atmosphere. It's just going to take a little more work to get there, if you even can. And it will take a more defined combination of parts including the turbo to get you there. For example, if you were to make 10psi max at altitude, you could go to sea level and make 12 psi max. This is where the power gain comes from. That extra 2psi worth of compression is available in the atmosphere.

For 600 Hp, that's really pushing the strength of the block some would say. And yes, likely the strength of the crank.