You can probably get away with mounting a short whip antenna out on the spreader (depending on the size of your mast and spreader). You don't want the antenna to touch the shroud
, and if you can get it two feet or more from the mast the antenna pattern won't be too bad. You will see a few dB of directionality, but probably not enough to make a big difference.
But I would certainly recommend that you mount it at the masthead if you can manage it. I hope you noticed all the times I used the word "probably"!