Hi Nick,
The kit for the70 series is a discontinued part # BM17995. I located one in a dealer's stock. He was showing the last listed price at $441. Considering how long he had stocked it, he was willing to sell it for $375 with shipping.
I'm quite impressed with the amount of hardware included. The installation doesn't look too bad.
A cover is removed from the bottom of the tramsmission. and one from the back right next to the rear pto shaft. The pto gearcase bolts where the bottom cover was, and a new shaft is inserted through the rear cover. The worst part is a multi-part system of levers and switches to effect the engagement, pto braking, and safety lockouts. There are fairly good instuctions with the kit, however. Are you considering adding a mid pto to your 990?