Timing/CAM gear

I bought this vehicle someone has put a V8 Chevrolet engine in it, (carburetor) hei ignition.
I got the paper work on the engine with the purchase, it was puchased through a dealership 2014. As a crate 350/290 truck engine I had this extreme banging noise from the front. ( Starts and runs) except for bad noise. So I remove front of the engine and timing chain cover and two of the three cam gear bolts are hanging out.
So go to remove to check threads and re-tap, I have engine at TDC, I thought!
So upon inspection I noted dot on crank is twelve o'clock but so is the cam sprocket. I thought all Chevrolet's were twelve and six o'clock. So I rotate several times and watch rockers (1) sure enough after intake closes and both are closed it approaches TDC the dots are six and twelve. Did Chevrolet make some like this? Or is something off?
It is like the cam sprocket is 180 off, but it would not run if that was the case.