Jeff's 1000 x 800 Gantry Router Design & Build

[JeffZ]

If you use a certain size aluminium profile for the gantry (90 by 45 mm I think) you can mount the BK/BF ballscrew supports directly to it, which makes things easier.

My supplier does offer 45mm (B-Type) extrusions, but the choice is very limited.
Using my supplier, the only sensible option is the one you're showing, using 2 45x90 extrusions for a 90x135 gantry.

I prefer the 40mm (I-Type) extrusions. They're thicker, easier to tap, there's more compatible hardware, and I can get them in larger configurations, such as 160x80 and 120x80.
I don't mind using the FK/FF bearings, but you're definitely right that using BK/BF, bolted directly to an extrusion is both easier and more adjustable.