Ray, depends on what it was used on. That has adapters that will allow it to attach to various models depending what frame and attachment kit it has. But if it was on JD 140 or any 3xx series it will be a direct attschment.
If a 54inch Blade is from the Following Models it will Hook Up to your model 300
keep In Mind a few of the Models Listed will Probably have a Manual angle Kit But the Blade will still work on your 300:thThumbsU