Do the small block 400's need and externally balanced flexplate?

Unless you have changed the crank to an internally balanced style, then yes, they do.

stock 400's are external every other small block that I know of is Internal as long as its stock. people building 383's out of 400 cranks have to use the 400 balancer and flex plate but now days poeple using 400 cranks for 383's is all but gone! Unless they do there own mech work.

Any of the small blocks with the one piece rear main seal are externally balanced at the back of the crank. (counterweighted flywheel)

Any of the small blocks with the one piece rear main seal are externally balanced at the back of the crank. (counterweighted flywheel)

Really? I didnt know that.

Yeah he is right I remember having a Lt-1 96 short block and the flex was diff.
Im usaly thinking pre 86 when I think sbc because all mine are and have been two piece. So you all will have to forgive my ignorance but Rust is right!

I was just gonna mention that, b/c it took me years to remember, pre-86 engines!

pdq67