used the existing Y axis to cut out holes and countersink for the new plate. Small problem on the right side, where one of the flat nuts were in the c beam runner and it stopped it from moving over which caused some head scratching for a bit since it moved away again, so there was some mysteriously shifted holes cut, i re-ran it to fix the holes so a couple had some extra holes just off to the side, otherwise it fitted well. The tool plate is bolted all the way down to the black Y axis plate through the MDF.
added the trucks and the delrin lead screw holder, with the 3mm spacers. i milled these holes at the same time as the holes for the truck so everything is parallel or perpendicular to everything else. zero movement in the Y axis now.. you can stand on it and slide back and forth with zero deflection.
i did cut it with mostly face cutting vs side cutting so it took a long time to mill, just picked the wrong cam operation in Fusion360 really.
I may end up bolting the whole thing down to either a granite/marble slab or another piece of tool plate, that will make it even more rigid and square.