It sounds like you've got a good guy doing the work. You don't need rods for 900 hp, let alone 600 though, so don't worry about that unless you just like to burn money or have hopes of some day hitting 1000. To keep it super reliable, and easily hit 600 for a towing monster, I'd suggest the following:
*Pistons: either stock with the bowl lip cut down or HD (IH maxx force) AKA Elite HD, Rudy's ultimate pistons etc. (only available in stock bore.) Do ceramic coating and total seal gapless rings on them as well.
*complete upgraded valvetrain including valve springs, push rods, and upgraded cam (elite stg 1 or equivalent- no valve relief required)
*head studs
*new oil cooler
*balanced rotating assy. (essentially the equivalent of the Elite stg 1 long block
https://www.elitedieselengineering.com/product/14060001/EDE-Long-Block/)
*turbos: Elite tow-power, or tow power plus. If it's in the budget, I'd say go for high powers as they'll make your truck run really cool on the EGT's.
*Stock size injectors, but consider Air Dog II if you have supply issues to the injection pump. Air Dog assist wouldn't hurt either.
*upgraded intercooler (no limit, spearco, precision, banks)
*IC pipes. I suggest no limit
*A trans and converter that can handle all of it (Elite stg 3, or equivalent)
*ported intake manifold
*plus all the other bells and whistles to make it work, most of which you already have
*Wastegate (may or may not be necessary depending on how hard you run it, but good peace of mind.
