I read about a method which I am going to try on my yacht sometime in the next few months. I think I read about it on this site in 'fairleads', but I can't be sure.
With the mast un-stepped, attach the rigging to the mast head. There should be no fittings on the other end of the wire rigging, and it should be a couple of feet longer than the expected finished length.
Step the mast using the crane, then attach 4 block and tackles, one to each chain plate of the forestay the backstay the port cap shroud and the stbd cap shroud. The other end of the tackles should be attached to each wire about 5 feet up, via a 'stropsicle' strop using an 'icicle hitch'. See the riggers handbook for details of these.
Once the tackles have been tensioned and secured, you can release the crane and send him on his way. you can then measure and cut the over length wires at your leisure, and fit sta-lok terminals. When I get round to doing this in a few months time I will be using Sta-lok turnbuckles with toggles at the lower end.
It might be an idea to have a cam on the bottom block of each tackle ( like on a kicking strap / vang ) to make it easier holding the tension on the wires.
I hope this has helped.