Vin trouble

I don't have a title it's an old car so it'll be pretty simple to register where I live but for 200 bucks a bill of sale was good enough for me.. I was told it originally had a slant six that would explain the lightweight rear differential and why my 360 killed the ring and pinion gears
Could a 76 Roadrunner come with a leaning tower of power? 318 or 360, but not a slant six, at least not as far as I know.