Personally, I'd consider just changing out the 4 pin socket for an octal socket and plugging in your 5AR4 directly. You may consider this too much of a leap for your skills, but in all honesty, it's pretty straightforward.
This tube search site will help you understand the wiring....
http://tdsl.duncanamps.com/tubesearch.php
OTOH, there's a vendor on Epay selling pairs of these for $10.
Search for "Vintage Bakelite 8pin to 4pin Tube Socket Converter Adapter 274B 5U4 to 274A 5Z3"