Word of caution here on file fit rings with a tapered cylinder bore, you CAN'T fit the new rings at the top of their travel and have them be good at the bottom because there is a good chance they will be TOO tight!
Most cast iron ring manufactures recomend .004" gap for every 1" of cylinder bore, in other words on a 4" bore you will need .016" ring gap. Now the kicker, if you have a worn cylinder with .010" taper and you file fit the rings to .016" at the most worn area (hence the top of the ring wear area) and then the rings travel to the bottom of their travel where there is very little cylinder wear, the gap will get to smaller and wear out the new rings very fast from being to tight.