Although you should set the timing without the vacuum advance I have never owned a 289 (all were stock) that had any vacuum advance at idle. If that is the case for you then disconnecting it will have no effect up timing at idle. That is somewhat fortunate since an original 289 setup does not have a vacuum hose but a steel line threaded into the vacuum advance which is more difficult to disconnect.
Is it hard to do? No, as long as you have a timing light and I think its a 9/16" wrench. I always set the timing at idle since it easier especially since the timing marks on a stock balancer only go to 12 degrees. If you have the proper scale marking and a way to maintaining a high idle PaulS method is also valid.
If everything you have is stock the idle timing should be 6 degrees BTDC. If you look in the shop manual there are about 15 different distributors for a 289 with slightly different total advance curves. They are all supposed to have 6 degrees at idle except the HiPo which was 12 and the Thermactor cars which were Zero. The Thermactor cars were intentionally mis-tuned from the factory for emissions and I doubt that many people ever really sets 289 timing at Zero after the cars were sold.