Personally, I start getting nervous as soon as a signal leaves a board!
me too

spi can get that sort of distance but you need a cs signal for each slave

I wonder if you used a pair of those 3v-5v level shifters back to back if they could act like a repeater ?
could be a cheap solution

wireless may be an option
rfm69 modules and the like can get 300k bps over that range and a master slave setup is no too difficult