Why not pass along the 5 bytes directly, or even better IMHO, translate them into two-character ASCII encoded hex values (like "FF" for 0xFF). The benefit of the latter is that you only send printable ASCII characters, never control codes which might get eaten by the UART (like XON/XOFF chars) or by the receiving end software (like Null, EOF, CR/LF, Bell, etc.). It also allows you to insert end-of-line characters or spaces at will, to demarcate the 5-byte values. This also greatly simplifies the processing as plain text is scripting language friendly and human readable. Binary is neither.
The conversion on the PC side would be simple. I'm sure a perl guru could come up with a one-liner that would eat the hex-encoded ASCII text file and spit out another text file with decimal values. In fact, you could do it real time by using the serial port as the input file to such a script.
With that said, I have worked with data as large as 40 bits on a lowly 8-bit PIC (really lowly... a PIC16F55). Writing an efficient algorithm, in assembly, to convert a 40-bit integer into a 13-digit decimal on a chip without hardware multiply or divide, is interesting to say the least.