The Mar-K tests reports are a GREAT read. I ended up using the POR-15 on the underside and just used an oil-based stain (penofin brand) on the upper side (my 72 C10). I did not what the clear coat as it tends to yellow and or chip and peal after time. My truck gets a few thousand miles a year and sits in some weather while I'm at work which would encourage a clear coat to break down. With that said, you can't beat a well done clear coated show truck bed. The oil stain option that I went with will require a yearly (or every two years depending) wipe down with some oil to keep it fresh looking, but that is only an hour or two job.
Here's a thread from the 67-72 forum that discusses bed oil treatments that you might find interesting...
https://67-72chevytrucks.com/vboard/...d.php?t=712931