I've a 1986 F-250 6.9, 4x4 4:10 and factory 4-speed manual (T-19?) - truck is all original, 198k miles now.
A previous owner had installed a Gear Vender in 1992.
According to GV online chart, when in 4th overdrive, I now run at 3:22 ratio.
I can now run 70-75 mph and keep at/under 2400 rpms.
Yes, can convert/change over/et al with a different transmission = a LOT of work.
Original Post-er said wanted to keep auto tranny, pretty sure it said that?
With the C6 and a GV, give GV folks/factory a call and discuss it with them, as one option/decision maker.
Were it mine, I'd opt for the GV - keeps things original and nice n' clean...