Gerry, have you considered using 3 bolts and 3 coil pins for a more balanced assembly?
Welcome to the forum!
My first dual sprocket setup was a rear 47/50 combo. This will work fine and will allow you to use the stock chain length for both sprockets.
I don't think I would go that way either (elongated slots). Going forward I'd worry about the tire rubbing something towards the front of the swing arm.i'd prefer not to do that!