I have the same clacking noise on my 66 289. Ive adjusted the rockers/ lifters twice now. Even gave them an extra 1/4th turn. They still spuraticly clack. I have 60psi oil press. See'ins this car is 40 yrs old and I have to fix the manifold gasket and carb. I will change them. They are probly week or sticking. Check the push rods for straightness. You could try running some crankcase cleaner and or add some motor honey.
As far as the backfireing.. check several things, distributer in good condition, plug wires not arcing or crossed, timeing correct and stable if the mark jumps around the timeing chain is worn. Check for vacume leaks.
Could be cap and rotor, advance unit working?
