It basically just makes your 340 into a 318 power wise. This idea it will produce more torque cause of velocity maybe at the lowest rpms like idle to say 2500 rpm but I really doubt it would be huge over the 340 J or X heads. And not worth the overall power you’d be given up.
Engines Torque is basically engine size, VE% and CR and engine that high lbs-ft per cid are usually big flowing heads solid roller cams big carbs headers High cr etc.. But yes that stuff tends to move the powerband higher up in the rpm range especially on smaller displacement engines.
For a truck or purely street engine you want idle to 4500 rpm power, for a more muscle car streetable .9-1:1 hp:cid you want more 1500-5500 rpm.
So yes you can used them but it’s a step backwards in the power department. The trick for a street hp engine is to try to keep bottom end hp 1500-2500 rpm while Extending the useful rpm from 4500 to more in the 5000 to 5500 rpm range.
