The PLCP is partially created by software for this chip. Selecting between long and short preambles is done in Sending.
Please see the attached PDF: PLCP-explanation.pdf
The upper part of the PDF explains what the PLCP contains in the case of Clause 17 data rates, the lower part shows how the software should generate the 6 bytes.
You'll also need to look at the 802.11 specification, this is noted in the PDF.
Note that 802.11g draws on Clause 17 for certain data rates, so you need the Clause 17 PLCP for those. For the other PLCP header, refer to the specification, its contents is easier because the fields are byte aligned.
For a less detailed explanation of both, see the PLCP-transmit-order.pdf