Go for a trans now if you have that kind of cash all together.. Then you can do smaller (far less $$$ intensive) mods 1 at a time. (unless your not looking for more than a 100 or so more HP to wheels).
After that exhaust, then tuning (don't waste your time with anything but EFI live or HP Tuners...EFI being the overall better of the two)
I agree. Do the trans first. It's one of the most expensive upgrades but opens the door to a lot more power. The next expensive upgrade is the motor. Almost the cost of your truck lol.