If I was going down this track I would be thinking a lot about setting up a rebar cage and a couple of flats in mild steel bar that would be embedded into the concrete and leave you with a surface that could be drilled and tapped, as well as ground to finish dimensions. There are a few way to make a lathe bedways. What is in the plans for the concrete lathe is the result of cheapest and easiest ideas, not really the most durable or accurate. The possible solid backing and resistance to vibration that a heavy mass like concrete can offer will be quickly negated if any part of the lathe is flimsy and allows easy movement. Standard lathe construction of having a very solid bed with equally solid travel ways is the key. The bedways do not need to be "V" ways. Rectangular ways are perfectly workable. Just the beginnings of food for thought.