Question about Corsair RM650x *2018*

The Corsair RM650 *2018* power supply measures 8.6 cm x 15 x 16 cm. A power supply that has all the connections and cables you could wish for! Good choice.
The HP Compaq Elite 8300 desktop should have a year of manufacture around 2013 and has the Small Form Factor (SFF) case type. The case width (or lying height) is given as 10 cm. So theoretically the NT could also fit in. But all that remains is to try it out.
Digitec is very accommodating and unsuitable parts can be exchanged in their original packaging (unopened) without any problems. Only 1 : 1 will show whether the housing slot is large enough for the NT and whether the screw connection is possible.