looking to rebuild my 340

its just weird my 340 has a street demon intake on it with a street demon carb and i floor it and it just doesnt take off like it should.....my 5.9 cummins is much quicker and that doesnt seam right to me it has a cam in it just not sure what all they did to it before i got it...put it this way i use to own a 77 formula 400 with a sb 400 and muncie 4 speed it had 6x heads a torker 2 intake and a 750 eddy carb and that thing would scream and light them up whenever i wanted