What is the rocker arm ratio for the 2 valve and 4 valve. Maybe you heard a rumor and maybe the difference is in the rockers.
I don"t know that total lift would be all that important for this application, though I don"t have any experience with a Detroit. Do both engines use the same lifter diameter? It would be nice to map the cams and see not so much what the duration is, but how fast the ramp is on the cam as well as exhaust valve timeing.