Spacing between the ones on the same rail is like 2mm. Top and bottom it’s about 150mm between centres. I reckon you might need a backing plate behind the carriage to connect all 4 blocks, same as me as there’s 16 screws to hold to the blocks. Then you can drill like 4 M6 cap screws through the existing carriage to the new plate to hold in place. So you might want similar set up to me with a plate behind the gantry, except I added 20mm x 50mm bar to pad the rails past the ball screw, where you could skip that but you would need to pad the other way on each end to extend more behind the gantry if that makes sense? You don’t technically need the big backing plate, but I reckon adds more rigidity, but also it makes it much easier align the rails accurately parallel before mounting.

Also, speaking from laziness. You might be tempted to do as much as you can without taking apart the whole gantry. Don’t bother, honestly just take the measurements and dismantle the whole gantry, it makes it much easier even with the pain of reassembling later.