One hole is a dowel pin hole used on some 454's and the other 6 bolts aren't equally spaced-one is offset with a smaller angle and a larger angle-the others then have 4 equal spaces-with the dowel pin hole between a couple of them ------
its important for the torque converter to fit the recess in the crank EXACTLY as that is what centers it so it will turn the front pump smoothly-the mounting holes are ususally oblong a lttle to allow a little tolerance on the bolt circle however they should be equal at 120* angle

If the crank isn't balanced already then have it done with the flexplate AND a dowel pin in the rear plus an front dampner for internal balance--- if its already balanced without the dowel pin then don't use one