Has anyone used a GDP intake or similar(think crazy Larry has a version too)? Heard airflow to the back cylinders wasn't good stock so was thinking about trying to help with that.

No real gains for intake mods unless your making big HP. Even with nearly everything stock I've managed to reduce EGTs super low. Even with stock HX35W turbo and 150 HP injectors. You'll get more results out of proper final ratio and good timing. 

I have always been a fan of Crazy Carls Tunnel Ram.  I believe his is the original.  

 

There is a small increase in flow.  There is a casting block that restricts flow to the rear cylinders.  

 

you can see the restrictions here https://www.dieselautopower.com/crazy-carls-tunnel-ram-vp

 

 

If you want something that will get you power and adjustability sell the smarty and get and adrenaline

21 minutes ago, dieselautopower said:

If you want something that will get you power and adjustability sell the smarty and get and adrenaline