I have a 90 RS with the 305. I'm wanting to install a cam and was wondering if I can get away with a (lopey-idle) cam on the TBI or is I should convert to a carb set-up to gain better performance in the upper rpm band?

As desirable as the "muscle car lope" may be, lope and fuel injection don't usually get along too well without proper tuning of the PROM. If you want lope without the PROM burning, switch to carb. If you want to keep the TBI, either get a cam that is compatable with the stock fuel/spark tables (generally a smaller, non-lopey cam) of get into PROM burning.

Is the car Speed Density, or MAF?

ThridGen TBIs are all S/D.

Yes. If you want lope look at cams with a little more overlap than normal along with 112 or lower LSA's.

Picking a cam based on how it idles is kinda silly dont ya think?

Yup.

Also TBI can't take much cam before is screams " you bastard"!