1971 Malibu with new 454 engine. I suspect that the fuel tank and lines are full of contaminates. Just had the tank cleaned out. Questions: The existing fuel line is stock. How can I ensure it's clean? Air blast? Would it be better to just go "stainless" now with larger diameter line to support the 454?

I recommend line replacement.
I went thru this within the last year on my '72. Fuel system had issues from the car sitting mostly for the past ten years. Had trouble starting and quit running at times. Suspected trash was coming from inside the steel lines. And yes, I was running a large fuel filter.
Ending up replacing the complete system...tank, pickup, lines (larger 3/8"), pump, and had carb rebuilt.
Problem solved. :thumbsup:

Thanks Bill---I'll go with your good advice!!