My neighbor has a '91 RS, which has been mostly sitting, for the last six years. The gas in the tank is from 2003 or 2004. It'll start, but barely run.
The general concensus is bad gas. Can I just drain the tank, and put fresh gas in it?
Will the injectors need to be cleaned?
Carburated cars seem to handle old gas alot better...

And, how do you tell if it's a 305 or a 350?

 

Gas that's old tends to coagulate. It'd be a good idea that at the minimum you should drain the tank and replace the fuel filter. Get some fuel cleaner - Techron by Chevron seems to work the best.
Do that and hope that you don't have to pull the injectors to send the out for cleaning.

On the rear drivers side of the block right behind the valve cover it should say either 5.0L or 5.7L.
Another way that tends to be fairly accurate is look at the color of the valve covers. Silver was 305 and black was 350. Seems to hold true in most cases where the covers weren't swapped. Take that though for what it's worth.

 

If it's a TBI, most likely it's a 305 - factory didn't do TBI on 350.