I don't think Spanish Florida is feasible in the long term, as long as the USA exists, what with it's manifest destiny. Of course, it was American Independence that allowed Spain to reclaim Florida, as it had been taken from them by Britain in the aftermath of the Seven Years War. There is also the problem that Spain offered a lot of land packages after gaining Florida back, but it seemed only to attract American settlers who ignored Spanish officials... Spanish presence was limited to a few hundred soldiers. As late as 1810, remains of British soldiers (!) revolted against Spanish rule. It was also a haven for escaped slaves, which peeved the USA to no end.
So you need too:
1) somehow allow Spain to keep Florida in the aftermath of the Seven Years War, or perhaps it never happens at all. Whenever Spain ceded Florida to Britain originally, most of the Spanish settlers left and Spain never bothered to aggressively colonize the territory again, even after regaining it.
2) Spanish presence needs to be increased -- there needs to be something of economic benefit which might no doubt draw in Spanish settlers? Keep foreign settlers to a minimum.
3) Prevent the Americans from gaining independence, or butterfly away the Revolutionary war.
These would allow Spain to keep Florida indefinitely, although for how long, I'm not sure.