Honestly I would avoid having the beam break sensors so far apart. Depending on the type of Adafruit sensors you got they may not even register at that distance (I know they make 2 different distances but I'm not sure either go far enough for what you are thinking about)... just seems like a wider margin for error that could turn into a pain in the neck. This coming from an idiot that has experimented and wasted a ton of time with different switch options... arcade pushbutton switches, the traditional metal wire switches, and IR beam breaks directly on each ring (I couldn't be happier with these, just awesome). Your mileage may vary though.
My machine is a scratch build so I'm not sure how easy this would be to do on an original machine, but I've basically just got a padded backboard that runs parallel to the scoring area behind the holes, then another board along the bottom that guides the balls to the ball return. The balls go into a ring, roll down the backboard and then roll to the right to enter the ball return. Each hole has a beam break sensor on it, then there is a beam break sensor on the ball return. Once the sensors are all set up, you just have to make your software play nice with it (ie have a "gutter" check variable that counts the amount of balls that have gone through rings vs how many balls go through the ball return). If a ball goes through the ball return before it has gone through a ring, the software knows that the player rolled a gutter. Not sure what language you are writing your software in, but below is a quick paste of some AS2 script bits to illustrate the concept:
When ring is triggered:
gutterCheck++; // adds 1 to gutterCheck variable
When gutter is triggered:
if (gutterCheck >= 1)
{
// not a gutterball, removes the 1 that we added to gutterCheck when the ball went through the ring
gutterCheck -= 1
} else {
// gutterball if the gutter is triggered before a ring
Balls -= 1
}
I'm sure this post was probably clear as mud, but unfortunately I don't have any pics of this part of my machine and it's in pieces underneath a car cover right now haha.
edit: forgot to mention, having each hole score individually as opposed to the "drain" style gives you a lot more freedom for game types as well