I see. 366 tabs, that's a handful.
As a pure troubleshooting step, I suggest making 2 new copies of the Excel file, call one file FirstHalf, call the other file SecondHalf, and delete half the tabs in each sheet.
Even half of 366 is alot of tabs, you might want to go right to Quarterly versions of the Excel files.
Then see if you can open those in Sheets.
Once / If you get them opened, you can rejoin them in Sheets into one document. I understand that some formulas may break, but it should be reasonable to repair them after the split files are joined.
Karl, I understand that it used to work and now it doesn't. Google changes things from time to time, recently adding direct edit of Excel files for example. Did that new feature cause the sheet to break? Your guess is as good as mine. All you can do is hack at it until you get imported into Sheets and then continue forward in Sheets.
Talk soon,
Lance