How much horsepower and torque is the 4L60E (stock) auto tranny rated for? I plan on having around 425RWHP after I finish my swap.

 

It's more complicated than just horsepower IMO. Max rpms and vehicle weight are also important factors to consider.

 

^^^+1
A stock trans will also last much longer with street tires than with drag tires.

Also depends a bit on the year as GM made improvements around '98 for the LS1 engine and again '02.
Keep in mind that you are talking about 12 to 20 year old transmissions now, often with more than 100,000 miles.
If the trans was recently rebuilt to stock specs, a Transgo HD2 shift kit can extend its life under high HP. However some builders say that installing a shift kit on a worn trans can make it worse.

Tell us more about your trans (year, last rebuild, etc) and we can then give you some suggestions.

 

Subbing... I am curious about this as well.

 

Its not the hp you should be concerned with,but the torque output of your setup.A engine built with lots of low end torque will kill these units faster than a engine with say 500 hp but only 350 ft/lbs of torque in lower rpm ranges.This is why turbo and supercharged cars are so tough on these trannys. mrvedit, has a good point concerning tires. Shock load goes way up with sticky tires which tends to break hard parts faster.