sounds like a real potent power-plant you guys have there....
if you want to go with an auto trans, I'd go with a TH 400 with a good performance rebuild from a reputable trans shop and have'em put in a manual/auto valve body as well as an aftermarket trans cooler, you'll really need it. Just make sure the trans has a B-O-P bolt pattern, chevy pattern won't fit your 455. If the engine has a stock lift cam then I would stick with not much more than a 2200rpm stall speed on the torque converter.
if you want to go with a manual trans, I'd recommend a Muncie M-21 "rock-crusher". It's a rock solid trans as long as you don't mind a little bit of effort shifting gears...they don't call it a "rock-crusher" for nothing.
as far as the rear goes, you need to know what you have in it already...if you have a 10-bolt(ring gear), pull it and get yourself a 12-bolt rear and have a good posi installed...again have it rebuilt by a reputable trans shop. As far as gear ratios go, it's your decision. If you plan on the car being a daily driver I would recommend staying away from anything higher(numerically) than 3.27's with tall tires. to give you an idea...w/ a TH400, 2.73 gear ratio and 255/60R15 tires you'll be running 60mph at approximately 2500rpm, give or take 100rpm
If you want it to be a strip car that you can just drive to the strip and then drive home, you could probably go as high as 3.89's which will give you a sweet holeshot with a set of fat tires but you'll probably be running over 3000rpm while doin 60-65.
I don't know your budget but "Gear Vendors" sells an "overdrive/underdrive" unit. It's pricey but it bolts to the back of the trans and is electrically controlled but a switch. It's nice because you can activate it in any gear, essentially turning a 3-speed auto(TH-400) into a 6 speed semi-auto. IF you went that route you could have high rear gears and a decent top speed.