I have always built with the spine on top on my surf rods. I use Alvey side-cast reels and blanks in the 12-14ft range.
They are low mount with short timber butts.
I haven't built for a overhead/baitcaster style reel but assume I would build spine on top still for them as the rod will still load the same way. Just need to remember to build on top of the spine for these.
You need to be careful as the spine is sometimes not straight and the rod can tend to twist under load so you might need to allow for that in positioning your reel seat..
Hope that helps