Super 6 power possibilities

Ok wasn't thinkin to much. There us someone on craigslist selling a /6 claiming 450 hp thru a 390 4brl. He said the was some had work done didn't mention big valves just springs. Thought it was kind of fishy seeing he only wanted 600 plus a built 904. Tho it still would be worth it for the split iron exhaust and carb.