Just had a thought that might make tensioning easier.
Just like they ‘spin’ suspension bridge hawsers in place, strand by strand, so you could (with a little ingenuity) ‘spin’ your tensioning rope in-situ.
Take a much lighter Amsteel Blue rope and fix it with an eye splice loop to an anchor pin.
Now loosely pass the lighter rope ‘n’ times back and forth through the core around both the anchor points in turn, where ‘n’ is the tension multiplier you need to safely pre-stress the beam.
Then tension each turn to the desired load sequentially, temporarily locking each loaded line while you set up to load the next turn, until all the turns have been loaded.
Finally, for the last turn, fit a sliding grip hitch with a spliced eye over the rope (again the ‘Whoopie’ is ideal). Pre load the last turn and slide the Whoopie hitch down in place and fit the eye over the anchor. Then release the loading tension to allow the ‘Whoopie’ to retain the end under tension. You might have to over tension the last turn in order to allow for the tensioning up of the ‘Whoopie’
Derek