William,
You will be aware that on this forum when someone asks a question, we lesser mortals all pause and wait for for the authoritiative answer to come winging in from one of the experts. However, I have jumped in because I am about as unknowledgeable as you claim to be, but I think that I may be able to help a little. I have only a few seasons experience of actually owning a junk rig boat. Initially it was rigged for me and I set off to sail. At the end of last year for the first time I took the two sails off the boat and had them overhauled. This year for the first time I was faced with re-assembling the pile of bits. What I t hink that I have learned is: very little if anything is critical at this stage; get the rig on somehow and see what happens when you at least pull it up or even better you go sailing.
This is all out of my head because my only copy of PJR is on the boat and I am 300 miles away. However, what I suggest is:
Lace the sails to the battens and keep battens (if that is what you have) using a continuous line lacing with a lashing at each end. That is the quickest and simplest. That is not the arrangement that I would use if I was setting off on a long voyage because I am fearful of wear in one place undoing the whole batten. However it would do to get you going. Use the cheapest string for the job. In due course you may replace it with something more durable (3mm Dynema?) and/or use lots of individual lashings.
Certainly I would not cut the yard at this stage. I presume that your hallyard and yard haul parrel go to metal fixings rivetted to the yard. At this stage you can move the sail up or down the yard to get it to hang according to the principles that Arne has explained. (Someone cleverer than me will provide a link!) If you cut it to length now you may well find that you have to move the two attachments points. Also you may find you have a length forward which may assist with the peaking up using a downhaul or parrel. Also a bit of length does seem to help avoid some of the jams with the lazy jacks.
In summary, suck it and see.
Finally, my rerigging the boat this year persuaded me to pay for an electric heated rope cutter. Not cheap, but my life as a rigger was transformed.
Anyway, there you are. I have stuck my neck out. I await incoming fire (sorry, constructive comments) from those much more knowledgeable than me.
Best of luck. We will all be interested to hear how you get on.
Jonathan