I have a crush on my ex's best friend

My ex and I have always been friends first. We still are very good friends. He still has a "crush" on me. We were together for 3 years and he treated me bad. We were young and he wasn't mature yet. Still isn't. I told him itll never work out again. Too many bridges burned. He broke off out "engagement" to date a stripper. He wanted me back when he found out she cheated. I took him back. But He got mad when I left him the second time we tried to date, for someone who actually cared about me. He got a new gf and I could care less.  Now that me and my previous ex are not together anymore, and he's not with his gf, he claims to miss me again. But I like his best friend. We have a lot more in common and he's a lot more mature. Doesn't have to go past a crush. Should I tell his friend I have a crush on him? Or is that overstepping boundaries given everything we been through.