Have you tried putting some wide handlebars in there and a beater front wheel to get some leverage (clamp wheel between legs, gently rotate bars)? Also IME some headsets exert a little clamping pressure when tightened, if you haven't already it might be worth slackening it off.
I’m worried about bending the fork tips. I’ll clamp a piece of wood in a bench vice at my dad’s tomorrow and use it the apply the force up near the fork crown.
Amazing frame, will make a great project.
Have you tried putting some wide handlebars in there and a beater front wheel to get some leverage (clamp wheel between legs, gently rotate bars)? Also IME some headsets exert a little clamping pressure when tightened, if you haven't already it might be worth slackening it off.