Well, I was trying to understand RECFM=U since some times, probably my little labour can help you here -
1. If you specify RECFM=U for an input dataset, MVS will truncate blocks longer than the BLKSIZE and accept blocks shorter than the BLKSIZE. This allows you to truncate unneeded data from data blocks, if that's what you want to do.
2. If the blocks are less than 32760, which is the largest BLKSIZE value that you can specify in JCL, then the file can be copied as RECFM=U, BLKSIZE=32760. This allows all blocks to be copied intact without knowing much about the format of the input data.