You will need to dissassemenble the transmission completely to swap the output shaft as it is the first part to be installed.
The 4L60 can be built to handle the power - it needs some of the hard parts changed. Check
http://www.700raptor.com/ For > 450HP the following should be upgraded:
sun shell
front and rear planetaries
sprag
output reaction carrier
input shaft and drum
3-4 clutch
band (wide band)
direct clutch
front pump - need mod's for higher output
boost valves
1-2& 3-4 band servo's
valve body mod's to speed up shifting
Not cheap but can be done.