What to upgrade first? Engine, or drivetrain
 

Got a 04 2500HD with the 6.0. It has a lifter tick, throws Misfire and Bay 2 Knock sensor low input as well and need to figure out. transmission is getting screwed becasue it was drag raced and after i lifted, i hit the brakes hard, hard a loud clunk or bang, and then had a handful of truck to take care of. also leaking tranny fluid.

apart from either's issues, what to upgrade first?

6.0L V8
4L80E