RickD_99 I would be using the Celestron F/6.3 reducer/corrector and attaching the 290MC directly to the F/6.3 R/C.
If you are using the Celestron 0.63x reducer, you can adapt it to the T2 (M42P0.75) thread using the Celestron T-adapter-SC.
From there, you just use T2 adapters to reach the proper back focus to the sensor of the camera.
T2 is sometimes called M42 or simply just "T", but there are two varieties of M42, with 0.75mm pitch (the Tamron T mount standard) and the ones with 1mm pitch (the old Pentax M42 standard). T and T2 almost always impliy the correct 0.75mm thread that you need. But when you see "M42," be sure to check that they are P0.75 instead of P1.0, or you could be cross threading your devices (like the threads on your camera).
The back focus from the metal back for the Celestron reducer is 105mm (metal back to camera sensor). The Celestron T adapter is 2 inches (50mm) long, so the distance from it to the camera sensor is 55mm. This number (55mm) matches the Tamron T-mount standard.
You can find lots of T2 spacers at places like Agena Astro, although ZWO cameras often come with spacers that allow you to achieve this "55mm" too. Remember that it is 55mm to the sensor, not to the camera flange, and needs to also be compensated if you add a glass filter between the reducer and the camera.
You did not mention the variety of ASI290 -- the mini and the hockey-puck versions of the ASI290 have different flange-to-sensor distance. The ZWO documentation should give the camera flange to sensor distance for your particular camera.
Agena also sells T2 metal washers that are 0.25, 0.5 and 1mm thick that you can use to fine tune the back focus. The optics are imperfect, and you often have to custom fine tune the distances to get a completely flat field, although for the sensor size of the ASI290, it should not matter as much compared to a larger sensor. For your sensor size, the 105mm number from the metal back of the 0.63x reducer, plus or minus a millimeter or two, should be good enough.
Amazon also carries a variety of T2 spacers, but you are limited to pretty much low end, inexpensive stuff from SVBONY and Astromania. The Blue Fireball T2 spacers at AgenaAstro are much more refined by comparison.