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I'm not sure this is making progress.  Paul, if you want to 
use marshal, you already can:  the pack and unpack routines 
are exposed in Python via the marshal module.  Freezing the 
representation isn't a particularly appealing idea; e.g., 
if anyone is likely to complain about the speed of Python's 
longs, it's you <wink>, and the current marshal format for 
longs is just a raw dump of Python's internal long 
representation -- but the most obvious everything-benefits 
way to speed Python longs is to increase the size of 
the "digits" used in its internal representation.  If 
that's ever done, the marshal format would want to change 
too.

It's easy enough to code your own storage format for longs, 
e.g.

>>> def tobin(i):
...     import binascii
...     ashex = hex(long(i))[2:-1] # chop '0x' and 
trailing 'L'
...     if len(ashex) & 1:
...         ashex = '0' + ashex
...     return binascii.unhexlify(ashex)

implements "base 256" for unsigned longs, and the runtime 
cannot be improved by rewriting in C except by a constant 
factor (the Python spelling has the right O() behavior).