HELP with Flywheel differences??????

[Griffin]

I'm swapping from a 3 speed to a SM465 and need some help. I have the flywheel that came from the donor truck but 400's and 350's were available in the 1976 donor truck and i don't know which one came in it. I know the 400's are externally balanced and the 350's are internally balanced. I know there is a difference but how do you tell? is there a extra weight like a flexplate, dirrerent size, etc... Will the flywheel from the 3 speed work?