Just be upfront... use that good old "I need to tell you something"...... and then just tell her/him how you feel and you think that that you'd like to turn this friendship into a relationship.
Nothing makes me more happy to see a husband/wife who are also friends. You already know each other pretty well. You have a dynamic that works... now just turn it into something more ever lasting.
And if you don't do the above, it'll just continue to eat you up or worse someone will come and take them away from you. And who knows, if you are really good friends, perhaps they're thinking the same.
Doesn't seem like you need GoMarry.com to find a suitable person to marry, looks like you've already done the hard work. All the best.