I have addressed the Baxalta / Shire transaction here:
https://ttlc.intuit.com/questions/3814587-can-you-help-baxalta-shire-merger-cash-and-stock
The transaction is fully taxable and you use the sum of cash plus FMV of the stock as "proceeds" to determine your reportable gain or loss. It's "as if" all the proceeds were paid to you in cash and then you used some of the cash to buy Shire stock.
Tom Young
