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Putting a stock 6.1 under drive pulley on a 5.7 harmonic balancer should work fine if the pulley mounting holes line up.
As far as putting a 6.1 harmonic balancer on a 5.7 motor, I am not so sure. The weight of the 6.1 balancer may not be optimum for the 5.7 motor. You may run into issues, but it might be fine. I would do my home work before doing so.
Things to check into would be the weight difference, hub size (going into the timing cover) so you don't have any leaks, crank hub diameter (will the balancer fit on the crankshaft) and pulley mounting holes. I have no idea about the 6.1 and 5.7 crankshaft specs (to mount the balancer).
I doubt you will have any performance gain from changing the harmonic balancer.
Some people do put under size pulleys as a performance mod. The under size pulley will slow down the rotation of the accessories attached to it. The water pump, A/C compressor, etc. This may free up a pony or 2 at higher RPM.
Not really significant enough to go through the hassle, expense and possibly have a negative effect to something else IMO. Engine cooling and battery charging may not work was well.
Just my opinion, I could be wrong.
2012 Tungsten R/T Classic, 6 speed w/STP, 730 RHR w/sound group II and heritage wheels.
MSgt, USAF Retired