-A nine weight may have enough backbone to get away with it, but for a casting rod you have two things against you (I learned this the hard way with a Sage 7-wt).
First - a fly rod blank is a fairly soft-medium action, even if it is a "fast action" fly blank.
Second - fly rods have most of their backbone very low in the blank, which means most of it will end up in your handle.
Neither of these are good for a casting rod.
I originally built the Sage 7-wt I mentioned earlier into a casting rod, that had too little backbone to keep the line off the blank, even though I put a lot of guides on it. I later stripped it and it is the best side-drifting small stream spinning rod I've ever used.