I've done 2x6 single?(I don't remember) t&g decking but not 3x6 double T&G. It sounds like the 8" spikes are to hold the boards to each other nail these in first. If you can find some GRK's missing the screw portion where it would be in the top board I would be tempted to try them instead of the 8# spikes. It depends on whether you have blacksmith arms. Then I'd toenail to the rafters followed by your nail through the back. Drilling the first piece for your 8" is also a good option. You can get some really nice 1/4" auger bits. We found it good to sit on the rafters with your feet and weight on the 2x6 we were installing to force it into place(or hang off it with your weight on your non nailing hand). Clamps like the one timberwrestler linked might be handy I've done similar with my nail bar. Decking the roof is often my favourite part of the frame.
