I don't know whether I agree with that assessment or not, although I DO agree that I see no good reason to stick a poly into an A just because you have one.
If you were to tear a poly and an LA down to the short blocks, you would have a VERY difficult time telling the two apart. In fact, the heads "nearly" interchange.
The real practical difference between a poly and an LA is the heads and intake.
I keep hearing people call these a "wide block" because of the large heads. The fact is that the blocks themselves are about the same.
Weight? I'd bet there is not as much difference as you might think
This guy claims to have actually weighed the important parts which changed:
http://www.moparchat.com/forums/showthread.php?t=117690
A partial quote:
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The LA (lightweight 'A' engine, lest we forget... ) only differs from the Poly in any significant manner in the heads and block. Crank, rods, timing cover etc are the same or interchangeable, while many other things (rockers, pushrods, cam) would be so close to the same weight as to be the same weight.
So I really only had to weigh a block and a head. Out with the bathroom scales!
Results:
Poly 318 block with main caps,
cam bearings in place,
main shells in place, original bore (3.91") .......... 192 lbs
LA 318 block with main caps,
cam bearings in place,
most main shells in place, original bore (3.91") ... 168 lbs
Poly 318 head with valves and springs only ........ 51 lbs
LA 318 head with valves and springs only .......... 48 lbs
Sum total difference is somewhere in the vicinity of 30 lbs.
Though this is ignoring another area where the LA might be lighter... the manifold. I really should weigh them. I know the Poly manifold comes in at 53 lbs, I can imagine the LA manifold is lighter
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