im confused here. it ran perfect 8 years ago with no changes its rattling after start up? Gas has gotten worse in the last 8 years but it should still be drivable.
Cam that will fix it if it was from cr is simple. Comp magnum 280H its a favorite of many hot rod guys its big its nasty but it should control knocking. But....
If it makes noise when cold its not detonation. its not easy to make an engine knock in the first minute of running unless you got something really wrong.
I am betting its a bent valve hitting the piston if I understand the problem right.
Knocking from high cr is between 1500 and 4000 rpm usally and only during hard acceleration usally. But if it was fine and now its knocking like crazy after setting for 8 years all the time then its not from CR it something much worse. Sounds like a valve got rusted open and bent when the engine was turned over.