If you have MT then you are good to go since it holds more max TQ naturally than AT, so no need to upgrade it.
If you have AT then your limit on a stock AT according to RJM id ~450lb TQ safely, anything past that is risking it.
I say get better and more sticky tires for the rear IMHO, and an LSD (quaife or wavetrac your choice), and really good suspension such as a set of KW-V3s.
Thats my opinion at least.