Hmm. Well, I'll think about it more but I'm fairly confident Modbus4J handles it properly.
Yeah you should be able to unpack the string the same way you packed it.
Edit: My recommendation, to prove what you're saying, would be to disable all the points on your modbus data source, the enable IO logging and the two points as 4 byte variants. Do the read, disable the points. Enable the 8 byte single point, do a read. Post the resultant IO log.