Thank you for your confirmation. I could see the situation on my local by using byfn of fabric-samples on fabric v2.0.0-beta. (But I haven't understood yet how to decide which order delivers block to which peer).
If we can align the size of metadata on each orderer to a certain size regarding each block, I think we are still able to keep our assumption available yet.
Is it possible to estimate the size of SignatureHeader and Signature at most? If it's possible, by padding to the maximum size on each orderer, we can align the size of metadata of each block across orderers, I think.
If it's not possible, we need to consider going to the way of block-based (not blockfile-based) archiving logic.