Are you sure you have a Newertech G3 upgrade, and not a Sonnet? Sonnet was the only maker I was aware of that made G3 upgrades where the G3 upgrade was on the card that acted as a 90-degree PDS adapter. If you in fact have the Sonnet, you can put either the Apple HPV or the AV cards in it and they will work. Cables get a bit tight, and the 6100's lid might not want to snap fully closed, but it will work. Incidentally, Sonnet went out of their way to publish a web page disclaiming any responsibility for the HPV card actually working in their 90-degree adapting G3 upgrades in the 6100, although I never had any trouble with it.

The NewerTech upgrades I knew of sat vertically in the PDS slot, then had a pass-through bridge cable and bracket to mount either the HPV or AV card upside-down in one of the Nubus bays of the 7100 and 8100.

I suspect you have the Sonnet G3 upgrade and not the Newertech. (Does it have a purple heatsink?)