If the author reads this: decommission your project, please and safely destroy the cells. If you really want to do this project drop me an email and I'll help you as much as possible to do this safely (and with a different chemistry).
Matching cells is hard and cells that have been through a different life cycle, especially cells that have been turned off for safety reasons won't stay matched for long. That is because their internal make-up will be different and so they will respond differently to charge/discharge cycles with one or more of the cells taking the other ones with them. And please use a FLIR on the pack to determine if there are cells that are already 'hot' compared to the rest. That's a sure sign that something bad is brewing.
All it takes is one degraded cell and this project is a fire waiting to happen.