The sound comes from the duration and valve event timing so look around at the specs and find one similar.
You will find that the lobes are very different on a flat tappet and a roller tappet cam even if the specs are the same. Cams with the same specs will sound less agressive in a flat tappet engine in general because the valve opens and closes slower (the lobe may have the same height and duration but is still physically smaller since the 'ramp' to max lift is more gentle).
In addition to that, the engine size, head design, and valve size have a huge influence on the idle. Finally, larger displacement engines can take more cam and still have a reasonable idle.
I assume you're building a 289 for your '66 (?) If so, the stock firing order is different from the late model cars (351W order, 5.0 HO, etc). You may want to get a cam that has the 351 firing order to get the sound at the tailpipe to sound similar to a 5.0 HO. You would then just install the plug wires accordingly....
I have the Edelbrock 7122 in my newly rebuilt 302 but it has not been fired up yet so I can't make any recommendataions about it.... The Comp cam in my 5.0 has a seriouos kick butt idle though (see sig.).
To be honest, I think the early v8's have their own sound and I doubt that you will be able to reproduce the 5.0 sound unless you actually use a late model fuel injected engine.