If you already have a HV-CA-554 or HV-CA-571, it is really hard to connect them to a standard Coax connector like SHV connector. HV-CA-554 and HV-CA-571 are Keithley specific cables and connectors for only 2657A. If you have SHV-CA-553, then it is easier to connect the 2657A to the standard SHV connector, because it is terminated on one side by HV-CA-554 like and the other side by SHV standard.
I suggest the SHV-CA-553.
Link below for specifications:
https://www.tek.com/en/manual/model-shv-ca-553-high-voltage-triaxial-shv-cable-information-sheet-manual
Have you considered the 8010 High Power Device test fixture? The diagram from the user manual shows how this test fixture can be connected to the 2657 SMU.
Link below for for 8010 manual.
https://www.tek.com/en/default-accessory-series-manual-34
