Starting from scratch with nothing, the only difference is a $400 (or less) 340 block. So now the 340 is at $1600 and the 318 is at $2000. If the money is moved to $2500 instead your window just closed. Doesn’t matter if you have a complete 318 because you can use the giveaway 318 crank/rods in the 340 and get the junk pistons. Now the playing field is level, use same heads-intake-cam-carb, ($2500 vs $2100) will $400 make up that 23 cubes? Personally I wouldn’t waste my money on either, that $2500 would go into a 360, cheap to find and much less than a stroker crank for the 318. My 5.9 has all the good stuff and I’ve got maybe $600 in it and will be above 450 hp.
