GV is who makes the bolt on GV OD and that is who you would buy it through as they dont have retailers that I know of.
The GV is the easiest to install as you should be able to install it your self in a weekend less drive shaft as you have to have the GV installed to measure for the shaft.

As I said if you change gear ratios it will be x2 as you have to do both axles and unless you have done this before or have a buddy that has it is not something you want to do in your drive so you now have to pay a shop $$$$$

The last would be swap out the C6 for a OD auto like the 4R70W and controller.
You can do this swap but it is not a full bolt in and drive like the GV. Trans will bolt to the motor and I believe crossmember but you have to deal with shifter linkage hook up and you got some more wiring than the GV unit.
A buddy has one he rebuilt behind a 300 six running Snipper EFI in a 85 F150. He has about 300 miles on the new motor / EFI / trans and been working out little bugs but it is nice.

I have done rear gearing of axles and I would not go that way to lower the RPM
The 4R70 is a way to go if you have the time to make all the little changes by hand, like shift linkage, trans cooler lines and more wiring and you may need to rebuild the trans so add that to buying the trans and controller. You do get OD and lock up converter that is a plus.

I think if you add up everything for regearing or the auto swap and the work needed for either, the GV is looking pretty good in my book.
But only you will know what works best for you so look at everything and make a list of pro / con of each.
Because I have a manual trans I looked at GV and Advance Adapter OD and for me the AA OD did what I wanted better than the GV and was cheaper.
Thing is the AA OD works only for manual trans and is no longer being sold
Dave ----