I never understood the point of getting a spheros or gosa 8k as it is literally a 10k with less line capacity. I didn't keep mine because certain plastic pieces broke and I wasn't in the mood for fighting to find the right parts from Shimano.
The gears of the Slammer 6500HS should fit in a standard slammer, but Tunanorth will be able to give you a concrete answer.
Oops... yes, the original Battle was all metal... says so right on the box in my closet. I also have a Saragosa 6k, not an 8k. Good thing I have an ophthalmologist appt scheduled!
I have a Stradic 4k, Saragosa 6k, Battle 6k (size of an 8k Shimano), Saragosa 10k, Twin Power 14k and a Makaira 20k.