either side can do this with policy NAT (or at least i know that your PIX can). Create a new IP range to NAT the overlapping subnets to (on one side or the other) and use the new IP range in the crypto acl.
It sounds you like you probably need " static policy nat ".
Here's the link for static policy nat for 7.2, the commands are the same for 6.3 as well.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...