These should be fine on a 64-bit platform. file-system-info invariably
yields floating-point numbers with the bottom three bits zero, due to
how modern file systems are built (their sizes are multiples of powers
of two). file-attributes returns an exact value unless your file is
larger than 2**61 bytes (2 EiB), and once we get to the rare files
larger than that we should be using bignums anyway. (The same could be
said for file-system-info, of course.)
Sorry for butting in, but in reading this discussion I'm wondering why you don't use the high order bits of the exponent instead of the low order bits of the mantissa?
Sure, you'd need some bit fiddling (rotation and masking) to make it into a proper floating point number, but now you'll find yourself in a situation where almost 100% of floating point numbers will fit in a word with tags.
Regards,
Elias