The trans. would have to come all the way out to get to the torque converter. Not a killer job, but no fun either. There is a small steel access plate on the center bottom of the bell housing held on by 1 or 2 small bolts, removing that may help make the call as to which part is at fault t/c or trans. If you decide to buy rebuilt not used make sure they will accept your "core" trans and t/c in whatever condition you find them in otherwise you could pay several hundred more bucks than you were expecting by the time you are done. For what it's worth I am currently running an '04 transmission on our '00 V8 4x4 Tundra with no issues in 6,000 miles so far.
I have been running synthetic fluid and an external cooler however all bolt holes /wiring/ plumbing hooked up direct with no mods. Good luck no matter which way you choose, oh and for what it's worth ,our Tundra had 175,000 on the blown trans. when we got it. Scott