The 70A isn't "Aftermarket", it was standard on all capris after a certain date. The 85A Bosch was used on early build capris up to ~oct of 1990 I think, worth double checking that.
The spindle doesn't need swapped, just the pulley - at least that's what is said. I've never swapped a pulley before and have had minimal (normal) issues with the belts, ALWAYS carry a spare belt and the tools needed to change it
You can switch to the 70 Mitsubishi alternator, but you'll need the electrical plug for the back of it, and the lower pivot bolt, it's a different size. Not sure the specs on it I had a spare one from a mitsu-equipped car. I have switched several before, it's a bit of work, the mitsu units are easier to find and generally cheaper and they work fine, the 15A doesn't make a lick of difference