I think Scooby's on to something...that would be my next thought after seeing the pics and now reading I missed stuff in "the Kid's" concurrent engine thread. At 168K miles, I'd also conclude there's some wear. Plugs are clean over long term so I second what Scooby noted.
If you have no future use for the factory 8445 heads currently on the engine and you want to get rid of them, you might be close enough to Jason B. (turboNasty) to drop them off. His introductory thread highlighting the ported heads he is developing as an alternative to Champion (now that Tom's passed) was lost from the site's 2021 crash. maybe something to consider when the project is further along.
As far as turbos go power goals are important,
Know how fast you want to go.
Get into the new stuff.
Rebuilding old teck is a mistake.
The new stuff spools faster and makes more power.
If you cant afford a newer turbo sit the car and wait till you can.