I just saw that too... I got the link from my friend, and I was sending him back what I think I need and I noticed.... not sure if they are interchangable.
I'll call DD tomorrow and ask them. then repost.
if you want a built 3650, Pro Motion Performance in FL apparently does some. Im assuming it would be less expensive than a TKO swap, just a guess though.
but at least you have 2 options, the Viper spec and Cobra spec. I think* a viper spec would be better for someone with 4.10s or 4.30s who wants to use 6th gear as a low rpm cruising gear.
other than that I dont know enough about to gear ratios of each to comment any further.