I have a new-in-the-box Advance Adapters Rotation Ring kit (p/n # 50-8604) for the Dana 300, with the 3/4" longer 23-spline input shaft for use behind a NV3550 (or any flush output shaft tranny w/23-spline output).
Asking $200 for the Rotation Ring/Input shaft kit .... or .... will sell for $175 WITH the purchase of the flpped D300 I have in another post.