Here's my tip.

JUST DO IT! THROW ALL CONSTRAINTS OVER BOARD!

The voice comes with the time. Especially for self written material there's no one in the world who could interopret it better than you. That's why guys like Clapton and Knopfler started singing their own material. Sometimes the authentic feeling you can put in songs weighs so much more than a perfectly pitched voice.

Generally and this counts also for covers, it is the best if you understand the song, its message, and if you can interpret and express that. It sure takes some time to be able to do so, but hey, you are a guitar expert, a super photographer, a marathon man, founder of the best forum on the net, there's nothing you can't do Rob! Good luck with your plans! Looking forward to hearing the first takes. I am sure it will be very good.