Yep, go for it. And just to let you know, in my experience, any horse with the proper training and everyday exposure will pick up cutting pretty quickly. If my BLM mustang will watch a cow, I KNOW that Duchess will. My brother even had a TB that was a culled bucking horse from Robert Etbaur that would watch a cow pretty darn well after 30 days in a feedlot. Plus, although she is taller than most reining horses, she still has the basic conformation that is common to them.
It is never too late to teach an old horse new trix.