Jim I and Q, I am Scottish and it is difficult to distinguish between the "B" and "P" sounds depending on the softness of the accent. Poor Archie was the only son in a family with six daughters and although he was born in Glasgow, his mother was from the (Gaelic-speaking) island of Vattersay. His speaking voice these days is much modified from Glasgow and has more Edinburgh in it (to my ear).
This was his first recording released in 1968 and, after further listening, the accent could be a "B" or a "P" on beets/peats. He could be singing "your own gallows you must..." but I don't hear a "T" sound on the next word, particularly not a "TR.." word. Maybe "Ply" as in "ply your trade"?
I guess the only person who could answer this would be the man himself.