A PROPERLY built 4L60E can take some serious power, what usually takes a shit is the 3-4 clutch pack as it's not clearanced properly or it's the endplay...if not those then it's a lack of fluid flow at higher RPMs or just a complete shit build to begin with.
Sure the 4l80E is stronger but it's cost prohibitive by a good deal over a 4L60E. Nothing swaps "right" over. You need the trans, converter, program the PCM, swap harness, fab an x-member, flexplate, driveshafts, t-case, etc. You're looking at a solid $3k all said and done if not more if you pay someone to do it... where you can get away with a built 4L60E for usually $1500-1800 installed and out the door WITH warranty if you're smart.
In a heavy ass truck, I'd say a built 4L60E could take a solid 500rwtq before taking a shit. Torque is the enemy next to weight, if you have your truck geared very low that will help ease the pain the 4L60E will have to endure.
Do a search for my user name in the Performance section of this site, I posted up a few pages of the internals for a kickass 4L60E buildup.