Ok let me try to figure this out. Do you have the 551 (pair of side posts and a pair of top posts) or do you have the 555 (pair of side posts and 2 pair of top posts). From what it sounds like you have the 555, which should have came with two sets of posts and bolts for the posts to use on the IBar kit.
Are you talking about the side terminal size? If so that is like the standard GM side terminal, which is a 3/8". The top terminal sizes will be the same as the battery which is an M6 thread.
If I missed anything or you need any more info just let me know.
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