Ok, at least you got some supporting evidence. A broken ring might not seal up well with the addition of oil, though. I'd put some pressurized air in the bad cylinder(s) and listen for where it leaks (via intake or exhaust or crankcase) before pulling the heads; it will be a very good diagnostic step to make sure of where the problem is.
The timing could be messed up for the backfires; timing chain is suspect......I'd change it too as it is not all that hard and can be done in-car. Chain/sprocket wear retards the cam timing and shows up as sluggish throttle response. Keep your pushrods and rockers in the same order, just as a good practice. Keep the head gaskets and examine for leakage spots. Look at the ridge in the cylinder bores as an indication of wear.
