I'm 6' 1", and I hear ya on the length issues. It shouldn't be that hard to keep your belt buckle covered at a slightly above-average height. I've found the following to be fairly generous in terms of length without being mu-mu-esque in the other dimensions:
- American Apparel summer "loose crew" (TTS, but kind of inconsistent in sizing)
- BLK DNM (size down 1, wash cold, hang to dry)
- Silent (size down 1)
- Odyn Vovk (size down 1 or TTS)
- Acne limit (size down 1), although I found the sleeves to be kind of short and boxy
Maybe it's time for an "official slutty t-shirts" thread.