I guess you are using an ECSBPK05? You would use 6/3 for a 50A circuit, with the caveat that you need to check for voltage drop problems (probably not an issue). You shouldn't use NM cable in outdoor conduit. Use UF cable or individual conductors. -- If someone refers to herself / himself as a "guru", they probably aren't.
nunya, you are correct I have a Siemens ECSBPK05 interlock and a 50 Amp two pole circuit breaker which I have yet to install. I am trying to determine the cost of 6/3 UF with ground in 100 Foot lengths. During my search for UF I found a lot of 50 Amp extension cords from 15 feet to 100 feet long in the Recreational Vehicle section of Amazon.com. The plug and receptacle are the same as a 50 Amp Electric Range cord set. I saw a picture of a 50 Amp inlet using the same kind of plug but I have not been able to find a surface mount weather resistant version. I might decide to install the inlet next to the service entrance panel and use a 50 Amp extension cord to connect the service panel to the 50 Amp receptacle on the generator. I appreciate your advice.