340 performance?

a friend bought a 73 340 4 spd.Duster
Has a '69 340,cast iron intake,x heads and lumpy cam..not much else known about the car
It runs 13.8 with 3.21:1 8 1/4 rear and 25560R15 BFG ta's
i know this is very little to go on but any guesses on et with 3.55 and 4.10 gears?
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