Force installing is quite complicated. Luckily, there's plenty of documentation about it, mainly due to Apple's decision not to include the first generation Mac Pro in the Mavericks-compatible club. I think this is something to do with the EFI of earlier Macs being 32-bit and Mavericks only supports 64-bit.
Basically, it's not worth it. But if you're desperate, the Macrumors
forums are a good place to go.
plusnet FTTC (54/12)