Hey, so it's been a while, but I was being lazy.
However, I found the replacement parts I needed, so I wanted to post here to hopefully help someone else in the future who may search for things like "replace fast attach spindles with star" or "swap fast attach spindles for something that actually works well". Hehe.
Anyway, MTD part number 918-06991 or 618-06991 is a 6-point star spindle assembly that fits the 54" fabricated deck (and probably others) on the XT2. The bolt pattern for mounting is a standard 5" x 5" pattern. The pulley sits above the deck the same amount as the stock fast attach spindles, and the blade is held the same distance below the deck. The spindle is between 6.5" and 6 7/8" depending on the listing you find. They run as cheap as $20-ish for the aftermarket stuff to $50-ish for Stens or $100-ish for Cub Cadet. I'm trying the 8TEN brand, which are around $25 each.
Some of the replacement spindles come with the nuts for the blades, and some don't. I tried a cheap aftermarket 3-pack of nuts for $5 (they are 5/8" x 18 TPI) but the threads were not cut properly. The MTD nuts are $10 for a 3-pack so I ordered those, and they fit fine.
The pulley on the 918-06991 is not quite the same. The outer diameter thickness is 3/4" where the originals are 1". The inside thickness at the edge is 1/2" where the originals are 5/8". The groove depth on the 06991 is 5/8" where the originals are 7/8". This all means that the belt doesn't quite sit fully in the pulley. I don't know if that's an issue, but you can move the pulleys from your original fast attach spindles to the new ones (just be careful -- the fast attach are spring-loaded and will blow apart when you take the nut off). The splines on the pulleys are the same. Or, you can order new 756-05038 pulleys (Cub Cadet or aftermarket) for around $15-$25 each. I found a 3-pack of OEM for $56.
I hope that's helpful to someone. I'm looking forward to not dealing with that fast attach madness anymore.
