different 318 question

The 318 in my 1974 has a stock long block with an LD340 intake and a carb from a 70 dart 4 spd. (don't know the CFM) and dual exhaust. With the original 2.45 gears it seemed to run fine, very low RPM. Car now has 3.55 seems to run out of steam after 3500rpm. The question, was the motor built as a really low RPM motor by Dodge? Will changing the cam make the difference? Read somewhere here that a K20-417-3 from comp cams was a good cam for stock motor. Thanks