Ash and asp (poplar) were much used, though the poplar used then (most likely Black Poplar which is almost extinct hereabouts) is not the same as the poplar in current use.
For a heavy shaft ash is good and poplar probably better for a flighting shaft than ash.
Another good option for the heavy shaft would he hornbeam. It should be noted that Ascham cites a number of shaft materials of which Ash and Asp are merely the most commonly used.
For a heavy shaft hornbeam is probably superior to ash in weight at a given dimension and was very common as a coppiced wood in parts of the country, having many "industrial" uses.
On the whole I think it likely that for mass production planing shafts from straight grained wood might be a more productive process than gathering and straightening shoot shafts.
As for spine, as long as stiffness is sufficient that the shaft will "stand in the bow" then it is not too critical.
Lets face it, at full draw you will not be looking down the shaft anyway, unless you are strong enough to anchor low enough to get your eye behind the string.
Otherwise your eye to mark and through the shaft to the mark are two separate and converging lines.
The aim will always consist of an intuitive allwance for offset, so having an arrow that shoots a little stiff is of little consequence, though it might be useful is the arrows were a reasonable match to each other.
Rod.
