Just because there's a corporate relationship doesn't mean the motors are designed to interchange. The Chevy small block is the more common swap choice for the RX-7. There 5.0 is usually swapped into Miatas because they have a smaller engine bay and it's a more compact motor. That's only for first gen Miatas though, because the 2nd gen are stuck with OBD2 and most states require a functioning diagnostic port. There's no ECU out there which will run both the 5.0 motor and the diagnostic port.