Body lifts on a diesel can be bad because of cooling issues. There are many people that have suspension lifts with no issues. Don't worry about bigger tires, most people put them on and have no issues. There is a TSB out for having bigger tires with the allison, I feel it is BS because the hummer h1 comes with 37" tires on it. Aftermarket pipes do not cause black smoke. People running crazy tunes do. If you have good programming, you won't see very much black smoke if at all any. The rule of thumb with the programmers is to keep it under 100hp and drive sensibly to not roast the tranny. The tranny will save itself before too much damage is done by going into limp mode.(failsafe) Lastly, a cat is the catalatic converter (sp?)...emissions crap that restricts the exhaust and raises exhaust gas temps (EGTs).