Bart, there is no such thing as valve rings. Valve seats, valve seals, valve guides perhaps, but no rings. Rings generally refer to piston rings. Now, exhaust smoke can be black and it can be blue-grey-white. Black, sooty smoke is too rich a fuel mixture and your carbs need work. Blue-grey-white is burning oil and you have engine problems. Do a compression test on your cylinders to determine if you have piston ring problems. You should get even compression numbers from all six cylinders. Squirt some oil in a low compression cylinder and if the compression jumps way up, its piston rings. If the compression reading stays about the same (with blue-grey-white smoke), suspect the valve train and do a valve job. Your explanation of back-firing sounds like carburation and fuel delivery. I assume you set the timing correctly from your description of ignition work, but bad timing will cause a backfire. Check your spark plugs after running the car and shutting it off from steady revs of 2500 or more. Are they oil fouled or black and sooty? Spark plugs should look light brown. Oil fouled plugs will look oily and grimey. Fuel fouled plugs will look black and sooty. After sitting for such a time, the car may have a number of problems, but from your description, I would thoroughly clean, tune and sync the carbs. Finally, make sure the timing is correct.