Hi Eric, and welcome to the VMware Communities!
This is purely an issue of CPU features. None of the processors Apple shipped in the Early 2008 Mac Pro systems support Extended Page Tables (EPT). According to everymac.com's list of Mac Pro systems, the processor you have will be E5462, E5472 or X5482, and the relevant pages on Intel's website (as linked) show no EPT support.
Without EPT, we cannot virtualize VT-x. In other words, the guest OS will be unable to see and use hardware-assisted virtualization, although Fusion itself will be able to make use of the host's hardware-assisted virtualization features to run the VM: We can use VT-x, but cannot not virtualize it to make it available to a nested guest.
Hyper-V depends entirely on hardware-assisted virtualization (e.g. VT-x), so it will not run as a nested guest in your configuration.
Fellow VMwarian jmattson has produced an awesome write-up on the requirements of nested virtual machines and the functional combinations here: Running Nested VMs
Cheers,
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Darius