The 410 and 428 were externally balanced for simplicity I would imagine. It was easier than trying to get the weight added to the crank and fit inside the block. So they just tagged it on to the flywheel/flexplate and added a hatchet weight behind the damper on the SCJ( for the extra weight of the 427 style rods).
The comment about the 360/390 rods being different and pistons the same doesn't add up dimensionally, the book I have in the house list both the rod and piston being different. I will look in the other material in the shop. It is unusual for a manufacturer to produce 2 different irems when one will do. I was sure that the rods were the same, for manufacturing costs, and that the pistons were different. Since I don't do many 360s anymore
, I am operating from my memory of 30+ years ago, so it may be fading,LOL.