Perhaps there is a work around, but the when I look at the spreadsheet, it is for making Custom Models using a single string.
You are looking at 64 strings, so I don't think it can help you. When I looked at the XML for my custom models (candy canes), they have no provision for multiple strings.
You may just need to make a Matrix with 64 strings and set the length of the strings as the "longest" string you have.
It won't look right in the Preview, but it might work for you. Perhaps use "Individual Start Channels" to help control your starting points.