Turbo and boost set up and head problems

[Slammedpete]

Alright guys I need some help. I currently have a 2007 Peterbilt 379 with a 565 ISX. When I bought this truck three years ago it had 600,000 or so on an overhaul done by a {completely horrible} shop in PA. This shop also deleted, tuned (was told around 600hp to tires) and added {puke-bits} {garbage} intake and exhaust manifolds and a {puke-bits} Big {garbage} turbo. Fast forward to 750,000 on the rebuild and my turbo goes out. I call said shop as I am in MO and told them I couldn’t find a Big Boss replacement and they told me to throw a Borg Warner 135/80mm on it and let it ride. Around 50,000 miles after this I had a piston end up in the oil pan. So my shop in MO decided we should just do an drop in 565 Cummins Recon in. We left everything the same and swapped my turbo, manifolds, and all the bolt on outside the motor parts. Fast forward to 100,000 miles on the new motor and I’ve got a cracked head. Cummins replaced head under warranty and 3000 miles later it pops again. They say it was a mis installed non ferrous ring. They replaced ring and told me it was good. 100 miles down the road and I’m dumping smoke out of the blow by and oil fill. Take it back and they replace liner piston and rod, rod bearings on cylinder two and new rings on all pistons. 20,000 miles later my truck won’t idle over 500 rpm’s. Chose not to deal with Cummins again and had my shop tear into it. 4 burnt exhaust valves, two leaking injectors, needs a new head as mine can’t be shaved and still be in spec. Cummins warranties the head again and we are in the process of putting it back together with all new rod bearings, new piston and liner on #1 cylinder, new head, and all new injectors.

So now my mechanic is recommend we turn the motor down to 550hp and 1850 torque. My question is do you think the bigger turbo pushing 42psi of boost is part of the problem or is my tune that ran on the old motor for 800,000 miles and 100,000 on the new averaging 6.7 mpg somehow now causing issues, or have I just had a run of bad reman heads from Cummins?