Not sure what your trying to achieve here.?

If you have Cast iron surface plate why do you need the lower frame.? Just bolts the rails to the Cast iron top.

The gantry and Z axis setup are both very bad idea. The rails on back side of thin plate will flex and resonate. But worse is the length of the top and bottom bearings plates make long lever that will flex like crazy. It will affectively be like putting Z axis and spindle on end of diving board.!!

Suggest you scrap the whole tall sided Gantry idea and go with better setup of building up the sides from the base to give gantry clearence you need and sit rails on top. This can be done with steel tube or aluminium profile.
Then gantry again made from tube or profile sits directly onto the bearings giving a very strong and simple setup. To give you some space back make bearing plates that offset the gantry back slightly and mount the rails on top n bottom. Put ballscrew on backside with Z axis top bearing plate extending back so can connect with ballnut.

This setup will be much stronger and easier to build.

Also forget buying Ecocast or ground tooling plate with these rails because the base of them will have more error than normal aluminium plate so it's wasted money.