You should be able to tow with the edge on level 1 or 2 maybe, that is kinda weird. as to the shim this should answer your question
2004.5-2006 LLY & LBZ 6.6L Release Valve Shim Kit [Shimkit] - $9.99 : Pacific Performance Engineering, Diesel Performance Technolgies, and the aftermarket fuel pump probably wouldn't do you any good since the engine is not demanding more fuel than the stock pump should be able to supply, but if you do go that route it will defintely be able to flow more fuel if the programming demands it.