Hi Dave,
I have the HFGH 10x12, and the part numbers do vary from your 6x8, but I believe the corner posts fit together into the frame the same way.

Here's a link to the part of my blog that shows photos of my framework going up. Scroll down a bit to see some close up pics of the corner posts, and maybe you can see from my pictures what you need to know.
Part Three, Assembling Greenhouse Walls

Sometimes the pieces in the HFGH kit don't go together the way you think they would, intuitively (one reason we wrote a blog to help folks with the awful HFGH manual.) It's very confusing. However, I have yet to hear of a single case where the kit parts have been miss-drilled, if that's of any comfort.

If the photos in my blog don't help, post back and I can take more pics of my greenhouse, but I'm hoping you'll find some help in the blog pictures.