Cast or aluminum intake?

I would use an Eddy aluminum RPM performer air gap on that motor.....Although the cast iron 340 manifold was a good design , the Eddy is much better and weighs about 1/2 the weight of the cast one.....If I remember correctly the cast one weighs about 55 lbs. and The aluminum around 27 or so....Your machine shop guy was right...probably about 20 HP + reduced weight savings....WIN WIN