quote "Why would you ever want to make your pistons heavier ?"
If the new pistons are lighter than the originals and you wanted to eliminate the need to spend big bucks balancing the entire rotating assembly, you would add enough weight to the new pistons to make them equal the weight of the original pistons. That's why you would add weight. If the new pistons are heavier than the originals, you lighten them to acheive the same weight. Simple.
According to a paper an old Chrysler engineer wrote, the reason the 340-360 engines are externally balanced is because they use the same connecting rods at the internally balanced 270-318 engines. I don't know if the rods were very expensive to manufacture, machine, design new ones or what the reason was, but supposedly that's why they had to externally balance the flywheel/torque converter and harmonic balancer on the big LAs.
It could be the same reason the '58 Chevy was a one year only model as opposed to the more customary practice at that time of having at least two model years using the same body stampings. Ends up the front doors were the most expensive body part for GM to manufacture on any of their cars. Starting in 1959, the same door stamping was used on the Buicks, Chevy, Olds, Pontiac and I believe the Caddy as well. When you have a part that requires multiple, complex stampings and assembly, you're better off making a bunch of the same one rather than fewer ones that are all different. And in actuality, the actual stamping or forming of the metal part wasn't overly expensive, it was the time consuming, very expensive process of creating the dies used to make the stampings. Beginning in '59, all GM divisions had to design their bodies around the same door.
Studebaker wasn't exactly flush with money and anyone familiar with the Studies knows that the bodies and front doors of the passenger cars were identical from about '57 through '64. There were minor differences in the roof line, hood shape, creases in the fenders and so on, but it doesn't take much to make a new stamping die for something with large, simple contours like a roof, hood or trunk. Or even the outer door skin. Take a good look at your average older car door and you will see that the inner portion has much more complex bends, curves, contours and holes-openings than the skin on the outside. All of those surface changes requries the die be machined to make that change.
So maybe that's why Chrysler used the same connecting rod on two similiar but different engines that had basically no other major internal parts that were the same or interchangable. In manufacturing, similiarity translates to less cost, which is why these days, all GM cars of every brand use the same, "GM" engine. Up until the very early 70's, each GM division designed and manufactured their own engine. Buick had unique Buick designed and built engines, Cadillac had theirs unique engines and so on. Today a Buick has a GM engine just like a Caddy has a GM engine and you can swap parts between the two all day long.