difference b/t flywheel and flexplate??

[67pro-street]

i am planning on getting a 383 stroker for my 67 chevy. it currently has a 350 engine and the original 4 spd. The guy ho built the 383 said that he built it for an automatic tranny so he said that i need to change the flexplate or flywheel (not sure what one) is this true?? can i just switch the one that is on my 350 right now with the one on the 383, or are the two balanced differently. Any help or general information would be greatly appreciates as i really dont know what im talking about. this is my first restoration so i am still learning right now.

thanks,
67pro-street