Dave, look for the head off a BOSS 1 to 5 but you will need the ram as well as that head is a really solid fixed head that doesn't suffer from the nodding and endless tramming that the rubber Bridgeports suffer from.

Best thing with the BOSS head is the quill is the ball screw. The balls run round the outside of the massive quill so no bolts and no trying to tip to one side.

This is one reason I posted in post #3 to look for one and not do the conversion. Don't know what the conversion has cost but got to be loads more than upgrading an existing CNC.

Hell for a grand you could have had my 10 x 54 CNC Beaver in fully working order with 4th axis, 30 odd BT40 toolholders. 6:1 speed increaser and 8" vise if you were prepared to strip it to get it out.