Well if the solenoid has been replaced and all vacuum line are good, my only guess is that the vacuum is
Shot. A google search will show home made mechanical wastegate with a spring. Very easy to make
First off your gonna need a boost gauge. Second you should do a google search for 6.5 mechanical waste gate or go over to diesel place .com. That site is dedicated to gm diesels
Yep it's that coil.
Changing up the exhaust like that really can't affect how it runs all that much. I did custom exhaust on my truck along with deleting rear o2 sensors and she purrs. No tuning either
Good call doing the head gaskets and head studs. 20psi will be no problem with that turbo. You should really consider gauges if you don't have them already.
Just noticed your in New York. Where abouts? I'm 30mins west of Albany
Yes, magnaflows or jones max flow.
I run a Jones max flow, very similar to the magnaflow. I love it. Currently running dual 2.5" pipes to the muffler and dual 2.5" pipes out. No need for a crossover with dual in dual out muffler