I'm sorry that I was unclear.
I would report the short-term proceeds with Unknown basis under one sales section total. Sales section totals may be reported with TurboTax Desktop or TurboTax Online and the tax return may be electronically filed. When you enter this information, date acquired and date sold are not reported in TurboTax.
The Online version will upload your PDF of the detail of the sales section total. However, I do not know what size limitation that TurboTax software has or the size limitation that the IRS imposes.
In either version, if you electronically file and mail information to the IRS, the software will generate an IRS form 8453 that is mailed to the IRS with form 8949 checked.
"The detail would include the sales date, the gross proceeds, the cost basis which you assigned to those proceeds and some declaration of how the basis was computed."
I apologize. My statement was intended to refer to the detail that you forward to the IRS and not the detail entered into the TurboTax software. The detail that is forwarded to the IRS would be in IRS form 8949 format and, likely, include multiple transactions lines that equal the total proceeds of $393,948.54. For each line, you may include a declaration of how the cost basis was computed, for instance, trade records or cost allocation if the individual trade records are not included.
Finally, you may elect to paper file the tax return and no IRS form 8453 will be generated. You may attach as much detail and information to the paper filed tax return as you feel is necessary to explain your income transactions.
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