If you are using Pi-hole's lighttpd, be aware that lighttdp.conf will be overwritten on Pi-hole updates.
For lighttpd version 1.4.46 or above, you should be able to overwrite the port from Pi-hole's lighttpd.conf in your external.conf like this:
server.port := 8080
Particularly note the := operator (which isn't supported in older versions of lighttpd).