Hey just bought a new bike, the stock seat post is just a hair too short. It's a 250mm post. All the other "road seat posts" are 250. Only the MTB seat posts are 350mm. I would like to try a 350 would this be okay?

The dia. of the OEM is 27.2 Can I use a MTB post that is 350mm?

35cm of seatpost... sheez... Are going to have 10+" of exposed seatpost?... Is that even mechanically safe?...

Pretty common on compact road frames and almost all mountain bikes. A MTB seat post (with the right diameter) should be fine in a compact road frame.

road post commonly are available up to 350mm... Your bike doesn't care if it's a mountain post or a road post.

This isn't a compact road frame, I just need about 2 more cm's on what I have now. It's really close at the max on the 250, just don't feel perfectly comfortable.

When i get it set, I'll probably cut some of the 350mm post off.

I would probably not cut off the extra seat post as it will improve the mechanical strength.. 0.02$

Chances are you'd be fine with the extra 2cm on your current post, the post just nieeds to extend past the where the top tube meets the seat tube for structural reasons... the "max" on a seatpost is a bit of a joke, and a guideline more than anything since it varies so much from bike to bike. On a bike like a Kona or Gary Fisher where the seat tube extends 2-3" ABOVE the seat tube, the "max" line on most posts isn't sufficient insertion.