Apparently json-parse can only read 32-bit numbers, and it results in MAXINT without error for larger numbers. Luckily it seems to be a minor patch to expand this to 64 bit numbers. As attached for today's 10.6.3.
[attachment=0]json-64bit-number.patch.gz[/attachment]
Thanks for the change Ralph, will be added - was fine on 64-bit newLISP, but now also works on 32-bit newLISP.