Ideally relays would switch the lengths of ladder line.
When I first implemented the idea, I obtained samples of 12v OMRON LY4 4PDT relays and they worked well at 100 watts. They have an anti-arcing plastic shield between contacts. I ran them from my 12vdc backup battery during AC power outages.
When I bought my SG-500 amp I went to larger 24v PB KUP DPDT relays that I got for $2 each from an electronic surplus store in Phoenix. I ran them from a 24vdc power supply.
After a couple of AC power outages during which I lost my 24vdc and antenna tuning ability, I started using the knife switches instead of setting up a 24vdc backup battery.
Secondly, the Balun design uses RG-400. That seems a bit difficult to locate and kind of lossy.
I chose RG-400 for its high voltage characteristics - available from The Wireman.http://www.thewireman.com/coaxp.html
RG-58 can be used as seen from the following chart. With the relatively short length of coax used to wind the balun, I wouldn't worry about losses.http://www.karinya.net/g3txq/chokes/