Dont think anything's wrong with that Animal. Somoene just did what they felt like they had to to keep it on the road. 99.99% of people think Chevy and GMC is the same thing from the 1930s to 2015. Thats not even close to right until 1978 if i remember right, but people cant imagine them ever being different trucks.
You could live with it and no one but about 40% of us in the club (plus you) would know. Call it a GMC and treat her well. As long as she has a big block GMC V6 in her I'll back you to my deathbed
Or you can swap out the cab to a GMC. You'll need a engine hoist, a donor truck, and a buddy to help lift and settle it unless you're crazy like me. Then you gotta just say a prayer and start doin it, smashing toes and fingers with wild abandon.
She's a good lookin truck either way. Congrats on her!