I don't remember my results now, but the differences between the Poly 318 and the LA 318 are:
Exhaust manifolds (?)
Essentially that's what you're looking at. From memory the Poly heads are 18lbs each heavier, though memories are fallible and that might be 18lbs total. The block had some extra weight and that would vary depending on which block you have. The later ones without the ribbing at the back would be lighter, of course.
And, of course, the intake manifold difference can be readily dispelled using an alloy intake.
All of this assumes you have an alloy timing cover and water pump.
The big block heads, again from memory, weren't quite as heavy as the Poly heads. But the block, crank,cam, rods and pistons certainly outweigh those in the Poly.