When I'm putting forks together I like to keep the finish nice. And yokes tend to be tight. What I do is block the groove. Then screw a bolt in from the wrong end (yoke permitting) and gently push against that blockage. This way the yoke can be opened ever so slightly. This makes it much easier to install the leg. Keep it tight enough and it'll still hold the leg when you run of for beer/tea/tools etc.
Obviously if you overdo it then pang, yoke ruined. So be warned y'all.