Sounds like the board won't surf as you had intended it to. No shame in salvaging the blank and shaping something different.
You can certainly glass on a fin, that's cool too. And you get a lot of extra glassing experience when glassing on fins. It's a good thing to do at least once in your board building life.
You could use another longboard blank and 'flip" it so the nose rocker would be used for the tail (as long as your outline template fits in the nose area [which would be the tail of the blank when you flip it]) Some guys flip the US Blanks 98Y for nose rider designs.
Or we can custom order you a blank to whatever rocker you want. No problem.