They have been blowing up by the dozens weekly Gunny.
And I don't doubt that. You've read some of the silliness that goes on in some forums. Many people are generally stupid. You've seen the guys putting a .600 lift cam 248/252 on a 108 in an otherwise stock engine. They just want bragging rights at the cruise-in.If you were to build a 750HP nitrous engine and use the stock crank and rods, it would blow up too. Same with trannys. Cheap parts equals a cheap build and guaranteed destruction.
Good billet/hardened parts and the sky's the limit. Some of the Impala guys are doing 1.3 60' times. Not bad for 4500 pound lead sled with all factory options and a 4L60E. That's a shock load. ;-)
Your at the limit where your at now in the SS Impala with the 4L60e.
I'm only halfway to the limit. I've got about 550HP and my 4L60E is good for over a thousand. Many 4300# Impalas are running low 10's and a few are in the 9's with the ole' 4L60E. Lots of 3400# F-Bodies are in the 7's and 8's with a 4L60E.
A quality builder and quality parts are the key.
The Sonnax smart input drum is no longer a fail point. I have it and a billet input shaft. Shock loads aren't an issue any longer. It doesn't get much "shockier" than launching a 4400# Impala on slicks. They're just not failing.
Now, understand that most of these are street/strip cars. Many are driven hundreds of miles to the track, the air conditioner is turned off and they run it and drive home. I'm certain that a 4L60E couldn't hold a 3000 HP drag car but for a street/strip application with big gears the .7 overdrive is a nice feature. TH400's don't give that option.......unless you go with the excellent Gear Vendors overdrive unit.