Today, dollar for dollar, is the 318 faster than the 340 ???

mopowers said

How would the 390 perform with the 318 cylinder heads? Would there still be money left to replace those?

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If the budget allows, i would probably run an aftermarket aluminum head. You have to objectively look at how much it would cost to redo the iron 340 core heads too, compared to aftermarket. Then, if you want your iron 340 to flow as good, toss in the cost of porting an iron head too?
(The last iron heads i had done, machine shop bill was $900. Mine were already professionally ported maxwedge heads. No way would i spend that on ordinarry 340 heads, short of a number matching, date coded restoration. )