I am a fan of the McMahon style snaps. Many for sale are of questionable strength. Poor construction, sharp bends with deep tool marks, and who knows what material. I too have a pull to failure set up, 300# max. Granted like many things we discuss here on BD how strong is strong enough, for you.
If you demand the best, small to largest, Leadertec is the only source I have found. Above are the largest, 300# doesn't make 'em even groan. (the swivels
are sourced elsewhere)
Now don't give the old man "what are ya fishin' for blue fin or blue whale's?"
Just telling where to get what I was looking for.