Roney, John & Bob
Can I suggest that you discuss the squareness issue in this seperate topic?

Bob's system of stops on the x-rails is essential. In fact, we always switch on the motors while we hold the y-gantry against the stops. The gantry is relatively flexible, and it can be pushed about 2mm out of square if the motors are switched off.

We actually fitted adjustable stops at all four ends of the x-rails so that we could check for y squareness at both ends of the x travel. Our gantry is now tack-welded together as security against slipping bolts, and to give better grounding.

During the assembly of the y-gantry, squareness is only one of the (easier) issues to deal with. More important is to get the 2 y-rails perfectly equidistant. The V-rollers simply climb up the side of the rail tip if the distance is not constant (parallel).

Then you should also check if both y-rails are perfectly horizontal. i.e. that the gantry is not twisted. A twist in the gantry will produce different xy-squareness at different z-heights!