i'm having a hard time telling from the pic, but there is supposed to be a small lip on the top and bottom of the bushing "holder" on the subframe so there is not metal touching metal.
are you sure it's not all the way in already?
also, i found that the black pipe unions that home depot sells fit perfectly. I think you need a 4" union. The big ones only come in (i believe) 3.5" and 4". buy both and return the one that's too small
how did they remove the old bushes without making some kind of pull/press tool anyway? torch and chisel?