Billet input is over kill, UNLESS you are sled pulling, drag racing, etc.. at 500HP.
the biggest issue in these trans is the converter, so get a good converter. Single or triple is fine. If you think you HAVE to have a triple, than you are making more than 500 and/or sled/drag and putting the truck through the ringer.
Ive done quite a few of these trans, and a good converter and just a stock type rebuild with good clutches and steels and a few mods to the valvebody, and its good to go for 500HP.
My truck is in the 500HP neighborhood, but don't sled pull. Im hard on my truck and Im running the stock input, intermediate and output.
My trans is basically a 47RE, with some 48RE parts in in it. Oil pump, planetaries, but a 47RE valve body with some mods.
I ran it like that with stock converter, and would blow through the converter under boost/heavy throttle.
Yanked it, threw a single disc in, and Ive been good to go since.
Most of the rebuilds I do, are along those same lines: Good converter, clutches, etc...
Don't get me wrong, if you can afford the billet, sure, now is the time to upgrade. Go for it. That way one day when you actually need it, you have it.
Same with triple over single. If you can afford the extra coin, there is no harm in going that route and will only help.
The reason Im chiming in is, is because if you are asking how much its gonna cost you in labor for rebuild, to me, that's saying you need to budget your money.
Nothing wrong with that. That's why I threw a stock converter back in my truck.
Of course, it didn't take me long to realize that was a screw up on my part.
The ONE thing you HAVE to have, is a good converter.
I dare say, that if you yank the trans BEFORE it starts slipping on you, and you screw it up, if you yanked it, plopped in a good converter, and your trans cooler is up to snuff, the trans would last longer than you can imagine.
The converter is our biggest culprit in these trans. They suck, for lack of a better term.