Glenn,
You didn't mention which platform you're running WAS 7.0 on or which version of WAS 7.0 (Community, Base, Network Deployment)...
I am aware of the issue you're describing, as is IBM (www-01.ibm.com/support/docview.wss?rs=18...), but haven't had to venture down this path myself (none of our applications require a JVM larger than 1.5GB.) I have called this out to IBM early last year, but their stance was essentially "Yeah, we know but we don't have a time frame when the PMT will be available for 64bit...."
I would advise that unless you need a JVM larger than 1.5GB you stick with 32bit. Betting that this isn't an option and that the business is dictating you use a 64bit isntall, you might want to check out the InfoCenter for version 7.0. You can search based on edition of WAS 7.0 and platform
publib.boulder.ibm.com/infocenter/wasinf...
Good luck and hopefully this helps.
Erik