[Cool Hand Lurker]

I echo Brentwood Bob's warning on using the cinder blocks. They only have strength when they are positioned with the holes up and no strength sideways. Sometimes a tap on the side can collapse them, especially the way you have them positioned. Don't die over a crossmember.

As far as getting the new one in, cut off one of the horn extensions just in front of the crossmember where it will not affect the strength of the main frame. See if you can slide the crossmember into the other side first and then back to the correct position. You may have to cut off both horns though. I would cut them off completely with a hack saw so you don't cut out a large gap. Bevel the edges of the cut, clamp it back in the correct position and reweld.