If youre looking at making 300rwhp, a shift kit and cooler I think would be sufficient. Maybe add a corvette or billet servo and/or overdrive billet servo if the shifts still feel sloppy. These are all parts you or a shop can install w/o having to drop the trans. It can all be done just dropping the pan off hte transmission.

I say its better to build and not break, overbuild it if you're addicted to power like I am I once said I'd never do anything more than the basic intake/exhaust.....years later I'm installing a blower, cam...etc

 

also the guts out of a 4l80e are able to transfer into a 4l60e... but i would rather just buy one rather than do that...