I built mine in 2009 and tried it with the blades about 1/4" below the skids and then added 1/4" wear strips to make the blades flush with the sides. No problem digging in with either setting, I would not bother with making the blade depth adjustable as I have never experienced the need for it.
The important things to consider when building a land plane grader scraper (lpgs)
weight, at least 100 lbs./ft of width, 150lbs/ft would be better imo.
long skids, longer skids help true up the surface for smoother results, true with most straight edge uses.
rigid frames. You want the frame to be rigid and non flexing on a plane in order to shave the ground flat and true.
Gordon is correct in that you want plenty of clearance for the dirt to flow through without piling up. There have been a few cases where leveling a field that trash has been gathered with my design. Next time I build one I will make the necessary changes to prevent this.
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So far I have built two lpgs models one 8' and one 4', the smaller 4' has rectangular box tubing sides to reduce the gravel from being carried on the skids. It is a pain to turn around and have the gravel fall off the skids into the grass. Limiting the horizontal surfaces helps with that.
Some things I would change on the next build would be taller skids, thinking about 4x12 rectangular box tubing with the skids being 8' long. Wider width of 14' for better pasture smoothing but can still transport on a trailer with no problems.
Here are a few pics.
