"I understand that an NTSC television normally can't go much beyond 320 pixels" ... this is an exaggeration of the issue. Try playing back an old 1980s era VHS video onto a TV versus a higher quality source (e.g. broadcast TV or DVD) and you should see a noticeable difference in quality. VHS (not the revised SuperVHS) has a horizontal resolution that is approximately equivalent to 320 pixels (352 is often quoted -- that's the resolution of SIF (or CIF, the PAL equivalent), a digital format intended to replicate VHS quality, and used for example in the Video CD format). Theoretically, an NTSC TV could manage up to 704 pixels horizontally, but this seems rare.
SuperVHS has a horizontal resolution of about 400-420 pixels, and to my understanding this was usually fully displayable by most NTSC TVs, even when connected to the VCR through an RF connection, which I understand was by far the most common approach in the US market (I've only ever used it with PAL so can't comment directly, but PAL TVs could definitely exceed even the SuperVHS quality, and SCART was commonly used as an interconnect that provided better quality than RF). The 704 pixel figure is from 4SIF (or 4CIF for the PAL equivalent), a digital image format intended to be approximately equivalent to the best a standard definition TV could be. In reality, you're unlikely to ever see that, but it is at least a theoretical possibility (and can happen with a digital TV sampling an analogue input signal).
So it seems that an NTSC TV, at least if it is reasonable quality, ought to be able to produce at least 420 horizontal lines. The question is, is this enough for legible 80 column text?
The answer is probably, as long as your font is appropriately designed. 5x7 fonts are commonly used (e.g. in LCD displays on printers and similar equipment) and are perfectly legible. These fonts usually don't include horizontal space, though, so you'll need 6 pixels per character to match them, meaning you should be using a display that provides 480 horizontal pixels. This is potentially pushing the limits of what an NTSC TV can display, but as long as the quality of the TV and your connection to it are good enough, it ought to work.
It's worth noting that the comments to the question mention an Amiga 60-character mode that worked well -- 60 characters and an 8x8 font would suggest that this used a 480 pixel wide display mode and therefore suggests that swapping to a 6x8 font should get you 80 columns working well enough.