So, I got lucky. I can run my 3070 and 3900x on the CX750M with no problems. I ran a stress test for 30 minutes with no issue, and then just to test the limits I went ahead and overclocked both of them. Still no issues after 30 minutes.
So, if anyone else has this question in the future, a CX750M is more than enough to run a 3900x + Gigabyte 3070 OC