You can try initTimeout = 300, retryTimeout should be 0.
We tried a request with "sleep 65" at the begining of the controler code, it works.
The 503 error may not all caused by the timeout, check if you Rails log, see if the request succeed or not. Also check stderror.log and error.log.
Maybe you request need more memory to process, so increase the memory soft/hard limits may help.