You are right .  In order to get power cross feed you need to have the lead screw with keyway in it.  I just converted my

9c to a 9a ,but I didn't want the power feed cross slide so I skipped that part of the conversion.  The QCGB works great for threading and power longitudinal feed.  I do have to take mine off again to loosen up the gears inside the box.  You will also need the right gears on the end of the head stock to get the QC to work right.

Sorry for the delay in answering back on your question.  I was away at a Sports Show in Harrisburg, PA.  I will try to send you the information I have stored in my storage folder.  It tells you how and what to use .  The author is Fred Adler.  If I can't do that you might have to do a search for it. Here's what you will need from his site to convert a C to and A model.

20 tooth stud gear, 40 tooth stud gear, Quick change gear box with/56 tooth gear , Lead screw with/drive gear, Model "A" or "C" apron assembly, Cross feed screw (gear driven). This is what is needed parts wise. You do realize you have to drill a new hole for the gearbox in the hardend way to bolt up the QC.  One hole with counterbore for screw.  It's a little challenging.  He says before proceeding any further it is highly suggested to visit the following site: http://people.ne.mediaone.net/…..r/SBLathe/ and both download and print all the available files.  I can't download his directions from my computer to this forum , but here is his webmail address and I'm sure he will send it to you and help you more than I can.       Sorry I can't be of more help.  You can still e-mail me and ask how I did it.  I know the 20 tooth gear is important ,because I didn't have it in my set of gears.  I had to correspond with the seller and persuade him I desperately needed it and that I would pay for it and postage. He finally agreed and sent it to me.  I have only used lathe to cut 1/2 13 NC thread and it works great.