Im not gonna say its an awful idea. Anyone can ride whatever the hell size of board they like, but its just a bit of a risk in truth. Effective edge is likely going to be too short for your weight (im around the same weight), so you will likely end up stuck in the park or washing. Performance wise youre also going to "bend the fuck out of it". This will mean LIKELY (since i have no idea about the flex of the board - but assuming youre going for a softer or medium soft set up), youre going to have little snap out of it and youre going to find it a touch squirly on landings. Hell, youll probably snap it though before you get sick of it so why the hell not?
But when you do snap it on a shit landing, just remember youre too heavy for it before you start screaming about poor manufacturing
The other problem, which might not be a problem at all, is that youre likely going to find yourself pretty much only pulling it out on park days. I dont know how close you live to the resort, but if you tend to day trip from a 100-200kms away, chances are youre bringing one board based on conditions that you arent ever entirely sure about until you get there. And having a quiver, nothing sucks more than guessing wrong and being stuck on your niche ride instaed of your kill it all ride. If you live even further away and do week trips, you will simply HAVE to bring another ride because it is going to be such a niche board for you. Nothing wrong in that, i stress this, ride what you want, but accept that this board is not going to be your go-to ride and it will be the supplement to your main ride because if theres any powder and you want to play in the powder, that day is going to fucking suck for you on that deck
What i would advise though is that you maybe demo a shorter board early season (or borrow one of your mates rides - if theyll let you, theyll probably think youre gonna snap it
), and see how it feels. We all like different things so maybe you find its the solution to improving in the area you wanna go so why the hell not... but honestly at 170lbs, i cant go lower than a 155 without feeling like i have a kids board strapped to my feet, and even then id rather have a longer ride. Then again, i cant go higher than a 160 without feeling like its a canoe, so plenty of mind games at work.