Best Tool for Tongue and Groove Routing 2 by 6 Yellow Pine

That's a job for a table saw with a dado stack. On 1 1/2ths inch (2x) pine, I would plan to cut the groove in one pass, then two pass per board to cut the tongue. I'd make it a fairly loose fit to account for warpage. That's 336 passes on the table saw. It will be far faster to cut all that on a table saw than on a router table. That's a long piece to hold in place on a router table. and even on a table saw, I'd want a roller stand on both the infeed and outfeed.

One other problem I see with a router table, aside from a lot of time slowly feeding the stock, is that pine can quickly gum up your bit. A saw blade will not have as much of a problem, but will probably need cleaning after the project is done.

If the stock is warped, it's going to be difficult to get your cuts just right, so carefully select the best boards, no knots near the edges. If you don't own a saw that can handle a dado stack at least half an inch wide, you can use this as an excuse to buy one, but BE SURE THE TS CAN HANDLE A DADO STACK. I like the 8 inch dado set for this.

By the time you buy the saw and the dado set (if you don't have one now), you've probably run costs up enough to buy T & G stock already finished. If your time is worth anything, add that and you may find T & G stock is really a bargain. 336 passes on either router or TS is a chore I personally wouldn't want to take on.