Drob80

I'm in the process of doing an swap on my 96 impala and going with an lq9 mild cam 224/224 600. 114 Texas speed cam , long tube headers and was wondering if my 4l60 transmission out my impala will hold up .

performabuilt

iTrader: (25)

Much over around 300rwhp the stock 4l60e becomes questionable more so the heavier the car, They can be built to handle some serious horsepower http://performabuilt.com/blackedition.html
That being our best above of course far more than you would need.
As for how long the stock 4l60e will last with mods you mention hard to say and depends on how hard you play of course along with its present general condition.

mrvedit

The '96 trans is also a weak unit, likely with many miles.
GM made some strength improvements in '98 to handle the LS1's additional power and then more improvements (and the 4L65E) in 2003 to handle the LS2's additional power.

Considering the effort it takes to remove and re-install a trans due to to failure, I would highly suggest getting at least a 2003 4L65E. Better yet, get a quality one from a sponsor such as PerformaBuilt; it has the shift kit and everything else built in.
Keep in mind that when a trans fails, it can also damage the converter. Kind of a pisser when your "free" trans ruins your $800 converter.