I have a recent stock merger. I received $33/shr cash and new stock valued at $17/shr. For LTCG, must I show proceeds of $50/shr against a basis of $17/shr or can I carry forward the old basis to the new stock? Been a long time since I did one of these. Would hate to pay LTCG on the entire merger value and start basis all over on the new stock.

It should be treated as if you sold the old stocks and purchased the new stocks. So you must report the sale.
This will also depend upon who this is treated by your employer. Wait till you get 1099-B or any other tax form from the broker or employer.