0k got wheel off 700 sci has same part number as stock502,does mercury balance these components? The 700 rods are I beam type and others are 7/16 type ,could they weigh the same? The pins are way up on the 700 type compare to 502 pistions any imfo will help

The cranks are balanced for the total reciprocating mass WITH a standard "external balance" flywheel/flexplate and harmonic damper. In other words, "external balance" flywheels/flexplates/dampers are interchangeable. Crankshafts are not interchangeable.

Does m e rcury or gm do that balance job

Do "what balance job"?
​​Whomever manufactures the harmonic damper or flywheel or flexplate balances each unit to a known standard.
Whomever specs out the rotating assembly has the crankshaft balanced to match.
SOME common assemblies can be "production balanced" before assembly.
In other words, a GM truck 454 or 496 was typically assembled with production pistons (with each piston weighing within a min/max tolerance) with rings of a known weight, on rods with known big/small end weight tolerance. They were bolted onto crankshafts that had been "pre balanced" to a balance factor that represents the average mass of those components.
A custom engine, however, is typically balanced just before assembly.
pistons are weighed individually, and they have material removed until the heavier ones all match the weight of the lightest one.
Then the rods are weighed and the small ends and big ends are messed with until they all weigh the same.
Then the crankshaft is balanced to match those EXACT weights (not theoretical average weights).

For a 330hp 454, I would imagine that they used "production balanced" cranks at a GM motor division.

For a Merc 700 SCi, it may or may not have been individually balanced.