Personal opinion, but I think the 8HP70 is a phenomenal transmission. Enough so that even though I have a new T56 in my garage for my Duster, I still sometimes wonder if the 8A wouldn't be the better direction.
But there is a caveat. If your motor isn't happy cruising under load at less than 2000 RPM, I wonder if it is the best idea. With a 26" tire and a 2.71 rear gear, 65 mph is about 1550 rpm and 75 mph is 1780 rpm, almost matching a new Challenger with the "performance" 3.06 rear gear. You could run a 3.55 gear instead and push the highway rpm's up to and above 2000 rpm in 8th but remember it has a 4.69 first gear. That gives you a 16.65 overall 1st gear ratio which would be like running a 727 with a 6.56 rear gear on the street.
Even with the 2.71 rear gear it is about the same as a 727 with a 5.00 gear, so still pretty deep.
You could run the 2.71 rear gear and only use 8th gear at 85 and above though. The A9 in my wife's 200 probably has less than a 1000 miles on 9th gear because it is too tall, so the OEM have done that. Or you could run a 3.55 rear gear and start in 2nd, that would like a 727 with a 4.37.
Plenty of options, just pointing out something to think about.
