100 mm front hub, 94 mm fork dropouts, options? Simple as that. I thought I could He-Man the fork to fit the hub as it's only 3 mm on either side. Looks like it requires a bit of force and I'm concerned about breaking something in the long-run; either the frame or the hub.

I have a Origin8/Formula front hub and an old steel frame (1970s or so) that seems to be in decent condition.

Which option is best: reduce the hub spacing to 94 mm or widen the fork to 100 mm? And in either case, what's the way to go about it?

The front hub is the top one in the picture below.



EDIT
Turns out my problem wasn't spreading the fork. It was the fact that the axle diameter was too large for the fork dropouts. I filed the trailing edge of the dropouts and tried again to spread the fork to accommodate the hub. It fit without issue. Pictures below.
Sorry to provide ANOTHER answer, but I think this one might be interesting:


Take a rear (135mm) axle;
Insert two nuts, one around each side, untill they are less than 80mm apart. One of them might be inserted just enough to allow a small piece of thread to show up in the tip;
Fit the axle into the fork dropouts with the bike upside down, so that both nuts are beween the dropouts;
Start to "unscrew" one of the nuts, thus forcing the fork legs to get away from each other;
Unscrew it after a while and check the distance. It might take some time to figure out how much elastic deformation (preload distance) you'll have to overcome before actually making the new distance permanent.


This method is much more controlled than the lever one detailed previously, but it most probably will take much more time (although I think a rear axle and some nuts are easier to prepare than a large improvised lever system).

Hope this helps