offset grinding crank

You cant hinkt of it as radially. A V 8 is balanced by putting wieght 180° from the reciprocating assembly, or as close as possible to that point. Picture each rod as it's own, not the 4 throws roataing around the same point. It's the imbalance of each rod and the power stroke that adds the twist forces on the crank. Not one set of rods balancing the other. That's why most straight 6s, 8s, and 4cyls have the counterweights spread out more, opposing each rod throw. Same reason one internally balances. To get the weight as close to directly opposite the direction of force of the piston as possible. You are comparing taking .015 off the circumference of the journal, vs adding metal, and then removing .080 from one side, and 0 from the other. You've move the center of mass of that journal out by .040 as opposed to keeping it in the same spot and just making the overall mass less. Plus, running heavier rods vs heavier crank throws, the heavier crank throws is the better of the two evils. The resulting imbalance is the mass of the weld times the distance you move it, or .040, times rpm IIRC. So the mass the crank "feels" is more, because it's further out from the crank centerline by .040". The rod throw of the crank is part of the total mass the counterweight is reacting against. You may need mallory to get it in balance too, althought it depends if you can lighten the small ends of the rods a little, you could probably get it close enough. If it's worth a try for you, go for it. I dont think I'd try to make it that complicated, but that's one guys opinion.