Keep in mind I have a V6, not a V8.
I've been manually shifting my trans constantly and I've got over 100,000 miles on my engine/trans with no rebuild on either. The trans does have a shift kit. I usually drive around town with the over drive off unless I'm above 45 miles an hour consistantly. I NEVER shift from 2nd to 1st unless I'm at a literal stand still as this shift is hard on the trans even at 2mph. One of the main reasons I will down shift from 3rd to 2nd is the engine braking. I usually get 30,000 miles out of my brake pads at a minimum and the rotors never have a large lip on them. Also, before I changed my rear end out I would select my gears manually while driving around town because I hated the shift points. So I've got a lot of miles on mine and don't exactly abuse it but I beat it up quite a bit and it hasn't had any problems keeping up.
One tip for you guys who like to down shift, if you are going from D to 2, make sure you turn OD off first as it makes the shift easier on the trans.
When I swap the V8 in I'm going to put in a 5 speed as well, 6 speed if I got the cash.