Using 27.2 seatpost in 26.8 frame I'm building a bicycle for myself on a steel road frame using primarily used and salvaged parts. I measured the inner diameter of the seat tube to be 26.8mm but I only have on hand a 27.2 seat post. Should it be fine to use this post?

I'm only asking this because, to my surprise, the 27.2 post fit cleanly into the frame. No force was needed to insert it and, there's even a small amount of play when the clamp isn't attached.
If it fits and you can clamp it, it's fine.