Since a promissory note is held over time and not paid at time of sell then I advise the following:
If the purchase price is paid through the issuance of a
promissory note, the shareholders can generally report the
gain on an installment basis so long as:
seller is a cash based taxpayer
promissory note is not payable on demand
stock sold is not publicly traded
In an installment sale, selling shareholders report proportionate amount of gain as they collect portions of the purchase price.
So you will be showing your capital gains over the period of the note.
I hope this inofration is helpful,