There should not be an issue with this fitting on your 60" Swisher ZTR with the extension kit. You would want to maintain a reasonable speed since the amount of discharge coming from a 60" deck can be quite a bit, so you don't want to overwhelm the vac. The reason for the 54" remark really applies to 54" tractors, not zero turns. Since you need to purchase the extension hose kit as well if you are going to use this with a ZTR, there really is no issue, but since the extension kit is for ZTR's or was made with the intention of using with ZTR's, 60" tractors then become a question mark because the extension kit is not mentioned when using a tractor. I provided the link to the extension kit as well, if you have any other questions, please don't hesitate in calling.
Mow-N-Vac Extension Kit For ZTR