Snap-on make an easy out kit specifically for alun heads. i know its works cuz ive got a set at work. maybe you could hire a set or buy a set, i think they were about £65 though.
they look like normal easy outs but theyre flat bottomed and not got quite such an extreme taper.
and try heating it up a bit, hot air gun or a glow lamp (CAREFULLY!) on the manifold just to warm it a little....co-officient of linear expansion should mean that it will losen off.