Yup, what silver said. Cutting them to the top of the nut is about 3/8"-1/2". And no, headers don't clear any more than manifolds do. I've tried them both.
And no, the ARP studs kit is not really designed for the SC. Basically, the found a stud they already made that was about the right length and would work. The part into the block is metric, but the top side is 7/16". (Pretty sure on the 7/16" part, can't remember for sure)
I'd actually contacted ARP about this through the engine shop I worked at about 18 months ago. They played stupid like they had no idea the studs didn't fit right on the outside row. I talked with 2 different people and they gave me the same **** even though I gave them exact specs on how 'off' the outside studs were. Said they'd 'look into it' which to me means they're not going to do a thing because I'm the only one that ever called and complained about it. It sucks to have to grind 6 studs on a brand new motor.