Is it true James Paul McCartney Jr, is right-handed but prefers to play his guitar left handed? 
Not true. Paul is a natural lefty. Ringo Starr, on the other hand (so to speak), is a natural lefty who plays drums in right-handed configuration.
Related: the guy who plays Paul in the tribute band 1964 IS right handed and learned to play left handed for realism.
Actually, I think Ringo plays (or at least played, when he was with the Beatles) lefty - bass with left foot, snare with right hand - on a righty-configured drum kit. If you watch old tape of him playing, he plays kind of sideways, and cross-armed, in order to hit the snare over his left arm.
I played the same wya the first few times I sat behind a friend’s kit, until I realized no one would kill me for moving stuff around.
When I was 13, some friends were forming a garage band, and realized they lacked a bass player.
“Hey Hal, your birthday is coming up…ask your mom to buy you a bass”, they said.
A few weeks later I had my spiffy new bass and amp. I sat in my room for a month with my record player learning how to rawk out some Dazed And Confused, Smoke On The Water, Cat Scratch Fever, etc…
Once I had all this down, me and the band (“Woohoo! We’re a band! Dude!”) set up shop in the garage. I slung my bass down low, and prepared to rock out…
“Uhhh…Hal? You know you’re holding that thing upside down?”
Errr, no. I had no idea. But there wasn’t a damn thing I could do about it by then. And so it’s been for the past 25 years – a righty playing bass and guitar lefty. <shrug>
I had a similar experience Hal, only much geekier. I essentially taught myself how to play mellophone. A friend of mine stopped in to see how I was doing on it. He listened to me play, then said, “Nice job, but you’re playing it backward.” Most brass instrument are played with the right hand. I, a clarinet player, gripped it the first way that felt comfortable, left-handed. I eventually switched over to the correct way.