Where you start depends on where you want to end up.....
Your transmission will be in serious danger if you add any more power without upgrading the valvebody at least. There are a several companies that do the trans mods, your best bet is to call each of them and pick the one that suits you best. It is impossible to have a transmission upgrade brand discussion on here that stays civil or even useful, so instead of asking just do a search. There is a months worth of reading with a search. When you decide on what brand transmission upgrades to get the builder will tell you where to go to get the upgrades done.
What power level do you want to get to and what do you use your truck for? Very important information when upgrades are chosen.