You're better off with an E4OD.. rob one out of a 2wd big block truck from the pic-n-pull for a core or have it built by someone if you can, then get a controller for it. I've been looking over the best way to get an automatic overdrive for a while, and that's about the least costly way I can figger out how to do it...
...I'll 2nd that. And converter slip would get worse with the added load at lower rpms.
You can swap the E4OD gearset into the C6... makes it a "wide-ratio" C6 with 2.72 first and 1.7 second gear... funner to drive for sure, but still sliping torque converter.
There's another option that someone in the forum was working on... the late model V6 truck tranny (6 spd ?). The aftermarket bell and controller pushed the price up to around $3k as I recall, using a pick a part donor trans.
I have also done business with Jay at Broader, and he was just fantastic to work with. He was very helpful and patient with me as I worked thru some issues (caused by me) with my 1000 HP rated C-6 he built for me. The trans is a beast that has been in the 9's at 4000 lbs.
That kit has a fitment issues. I have seen multiple people be rather dissatisfied with it. I ended up buying the speed gems kit and it uses the v8 6r80 instead of the v6. I havnt installed it yet but it is machined very nice and has a lot better reviews. However it is more expensive. I have about $3600 in the whole setup with a junkyard trans. I found a 6k mile 5.0 trans from a 2014 f150 for 900. That price also includes the trans controller as well.