Unless you buy a long block from a reputable manufacturer, there's no guarantee what you'll get. Why not just rebuild what you have? You already have all the components and it's already marine-ized.
Buy an engine stand and take your time. Investigate what is causing the knock. Maybe it's not as bad as you think.
Getting something from a junk yard may require a rebuild, you just don't know it's history. You might get something worse than you already have.
You'll also need to do some work to marine-ize it, proper gaskets, components, etc.
So, you'll be taking that truck engine partially apart when you can put that effort into what you already have.
Some of your marine components may not fit the truck engine, then you'll have to cough up more cash to get the right parts. Your old engine has all of that.
I can see doing a swap if your block has substantial damage, but a knock doesn't seem to fit that bill.
I would start a tear down of the old engine to see it's internal condition before venturing out for a used unknown one.
