Small Block & Big Block Chevy Specific - flywheel question

my buddy has a 350 motor that's pretty much stock. it came out of an automatic car and we're throwing it in his truck tonight. we need to know if we can use a 400 ext. bal. flywheel or a neutral balanced flywheel from my old LT1

no. the lt1 is counterbalanced also, unless the 350 is a one piece rear main motor, then you can use the lt1 flywheel.

All 1 piece rear main SBC motors can swap flywheels and flexplates.