354 or 392? Hemi? Questions?

Ok well looks like my dream of having a good deal on this 354 hemi is gone.... He doesn't know much about it except it came in a old hot rod his father had... He said it says desoto!!! On the valve covers!!! Is it still worth it? Or no? I'm upset now because I was ready to have a 354 hemi in my duster. Now he says it's a desoto. Are they the same thing as far as swapping em? Power wise? Maybe I'll get lucky and it will be a big desoto