My '92 Trek 1420 came as a 7-speed triple and friction shift front down tube der. The chain rings and cassette are looking like they need replacing and I'm having a hard time sourcing a 7-speed triple crank. I have found some that indicate that they are 8/9-speed compatible though. Will they work ok with a 7-speed cassette? I'm not sure what the difference is regarding spacing between the chain rings but I assume the chainline should be the same. Also, what chain would I need to use? Thanks in advance for the advice!