Will Rod swap require balancing?

Why not get stock 273/318 floaters? they are already in the ballpark of the 318 castings (same) and wont require a balance to get it back to factory specs. The 318 pins in the pistons you have are not the heavy wall 273-2 pins. Factory specs were a bit loose to facilitate production so you dont have to be spot on to get it back. As for weighing rods, its pretty easy to weigh the rods split on a standard gram scale. The fixtures obviously get you a big end/little end weight on both ends but since the balance pads are at the extreme ends, weighing the split rod parts on a scale is very close to actually jigging them up as reducing weight on either end is not likely going to produce any noticeable deviation on the other. The lever effect of a larger balance weight on the big end effect on the little end in a fixture is so small to the <6000 RPM street motor. It is very hard to get repeatable weights on a jig wiithout a dedicated scale as the sensors on a cheap scale are 4 corner and any deviation of the angle or even the placement of the fixture on the scale will give you a different weight +- 2 grams. I think the $$$ professional machine shop balance scales are center weighted to provide a single sensor and will not deviate if the load is slightly off center. Mark the mating pairs and get all the caps to the lightest weight and then repeat with the little end side. The bobweight data the jig provides is for the balancers sheet but your only interested in getting the rods equalized to the original rods. they are probably within 5g already if they are stock parts.