Contrafactic history (which I deplore!) If he had continued into his 70's, like Mick Jagger, I think some singing lessons from a proffessional would have been beneficial. Singing or talking with this fantastic blues/rock voice take it's toll.
I'd argue either Irish Tour 74 or Fresh Evidence showcased Rory Gallagher's abilities as a vocalist reasonably well. I wish there was more discussion on his vocal abilities,Would make for interesting conversation.