Thomas,
8 mpg is low, unless you are driving the car very hard. I would expect more like 12-14 mpg from a standard 400cid engine - but that's an engine in good condition with a properly sized carb with the right jets.
Peter, you get poorer performance from your 400 cid engine compared to the 383 because the 400 has "smog" heads, which don't breathe well (by design to reduce emissions). If you put original 383 heads on the 400 you would see an instant improvement in performance. Alternatively there are aftermarket heads you could use, but they will usually be aluminium alloy so they won't look original, whereas it's almost impossible to tell between the original 400 and the 383 heads.
kevin
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