Miranda,
you're in luck

. I have a pair of Sidi Women's Blaze (cheap MTB shoes) in 39 and a pair of Sidi Men's Zeta Carbon, also in 39.
Both of them fit me. The men's is very slightly wider overall (including a bit more width in the toebox). The heel is definitely slightly looser. However, my foot is shaped like yours (wider forefoot, narrow heel), and it hasn't really been a problem for me, the fit is not sloppy. Next pair of road shoes, I'll probably buy women's, but these are fine for the time being.
I would say it might be worth trying a pair of men's Sidis, especially if you could try at LBS with no obligation to buy.
By the way, I've read that Sidi's men's and women's lasts are differently proportioned, and that the women's last is actually wider at the forefoot and narrower at the heel compared to the men's. So men's narrow is probably still a little different than the women's fit.
Good luck!