Don Terrill’s Speed-Talk

For a purely street/utility use 351w that was balanced with a set of 790 gram pistons/pins, would you rebalance after to going to a 21 gram lighter piston and pin? This thing will rarely, if ever, see 5000 rpms. I am concerned about long term durability and having a smooth running engine

You'll never notice the difference but if you can afford, especially if you need to replace a flex plate or resurface your flywheel or replace your harmonic you may as well balance. Much better than wishing after the fact.

Being lighter like that wouldn't even make me blink. Run it.

Lots of engines are deliberately 'overbalanced' by more than that.