[ul][*]What size would I need? #65-70lbs 27.5" arrow. [/ul]
I went with the 3-71's......since fps on the shots I take wasn't an issue out of my Ally. I'll shoot 3-71's out of my 8nd, too.....for the same reason.

[ul][*]How heavy are most of your finished arrow weights, with a 100gr head? [/ul]
My hunting arrow weighs 427gr.

[ul][*]How's your FOC with the heavier GPI shaft and a 100gr head? [/ul]
Clueless. Over my head. Sorry.

[ul][*]How is the overall durability of the shaft, given the fact it has the aluminum core?[/ul]
Last year.....in fewer shots at animals.....I broke probably 4-5 shafts. This year....I bent or broke none.