You could propably mount the single 300 in the center of your mount, but you will still have a big bracket on the back, exactly where it's not needed. For the $$$ you are investing in a Zuki 300, you may want to go with a single motor bracket, or even consider a hydraulic jack plate. This would clean up the back of the boat and leave more room for fighting fish, a swim platform, or whatever.
Bob's Machine is now selling a hydraulic jack plate that will work with a 300 hp - you can check it out at:
http://www.bobsmachine.com/Let's go fishin' and good luck!