Sinc Interpolation Using DFT (FFT)
Doing DFT based interpolation has to keep 3 principles:

Keep the zeros outside the data (Looking form $ -\pi $ to $ \pi $). Namely add zeros to the higher frequencies.
If it is real signal, keep the symmetry (If needed, divide the highest frequency into 2 items).
Preserve the energy (By the division by 2 if needed).

By the way, the interpolation isn't by a Sinc Kernel, it is done with its discrete form - The Dirichlet Kernel.
See The Proper Way to Do Sinc Downsampling (DFT Downsampling) for Uniformly Sampled Discrete Signals with Finite Number of Samples