I did that trip last year, but about a month earlier. Inside or outside depends on your draft and whether you want a liesurely passage or just want to get there. I sailed from KW to Marathon in one day, stayed a few weeks in Marathon and then sailed to Rodruguez Key and then to No Name Harbor on Key Biscayne the next day. I mostly had to motor into the northeasterly wind, but conditions would probably be better in late March. There are a limited number of anchorages in between those stops and even less access to shore on the outside.

Years ago I did it inside on a boat with a 5' draft and had no problems. Many more anchorages and places to stop. But a 6' draft could be a problem inside.

David