LQ9 6.0 exhaust manifold ports - oil burn?

La swap on my 1955 bel air. I followed 2carpros how to rebuilt 5.3 & LS swap vids. I attached three photos - cylinder exhaust port 3 & 7 are black compared to the others. This is from the initial start up. Only stated engine 4 time for no longer than 5mins each time I started it up. There was smoke coming from headers and smelled like rubber. Engine block and heads were sent to machine shop... I’m looking for your opinion please :). Is this leaking valve steam, position rings? Thank your review and help
Julian. Note sure if photos uploaded please let me know thank you