I did a similar thing. I bought a SATA adapter and then got the PCI SATA Card from MacSales and run that as my boot SSD. I have done similar things on two MacPros, it's not sexy, but it does reuse what would otherwise be parts sitting on a shelf. The SATA adapters looks pretty sketchy, but actually end up doing the job. I got mine on Amazon. Make sure the description of the adapter is the one for you SSD because Apple made slight adjustments to their SSDs over the years.