Denverd0n has it...Lake Monroe/Sanford is as far one would dare venture given your draft. I have seen some shoal boats in Lake Jessup. The fishing must be really good there 'cause you couldn't fall out of the boat without landing on a gator...they're HUGH too!
There are 3 fixed bridges that limit a sailboats mast height to 45', so your only restriction would be the RR bridges @5' to 7' but are in the up position unless preparing for a train...maybe a 1/2 hr. wait.
I've always wanted to make your planned trip, but have only been as far as Lake George from Lake Monroe. The closer you get to Lake Monroe the narrower the river, so be advised that although seldom, barges (think BIG) do transit the river to supply the power plant just north of Sanford with fuel. The river's bends can be 'exciting' with the bass boats moving from their favorite honey holes at mach 1.0, so one should never let their guard down in the twisty sections.
There is a lot of history the entire length of the river (pick up a river guide) so 'low & slow' is key. If not into the history, the scenery and wild life 'make' the trip. Sanford (lots of history there as well) is nice...lots of antiques and shops within walking distance from Monroe Harbour Marina St. Johns River
but the river makes it all worth the trip. The River Walk is nice to stroll at night and runs by the marina.
Only drawback to the trip is the tannins in the water from the decaying vegetation and cyprus that will leave some evidence of there you've been. So, wax her up and enjoy the trip!