OK. You know what you are doing. That makes a big difference in the answer.
The last GEP I had apart I used these guys for bearings, rings, gaskets and TTY head bolts:
DNJ ENGINE COMPONENTS, INC.
enginecomponents.com
That engine was surplussed and threw the #1 rod during a compression test. Turns out it was sold because it had suffered no oil psi and the bearing/crank were already destroyed before it was spun to test. I had a 6.5 with a broken starter mount that donated a crankshaft and other needed good parts. The resulting rebuilt GEP 6.5 runs good.
If you are going to add a turbo and go for power. My suggestion would have been contact Heathdiesel.com. Except going there just now I find they are changing names. Maybe Mr. Heath is stepping down. He built a 6.5 that set the land speed record for them around 160 mph a while back. Great guy to talk engines with. If he is still involved. Another source of information might be some of the marine/boat engine people. Low compression pistons with high boost and big intercoolers.