3 things.
1) what are the gantry sides made of? They look too thin. You'll probably see side to side flexing.
2) I'd use 6 legs, as you'll get sagging in the middle of the table
3) The extrusions are only as straight as what they're bolted to, so you'll need to either get the steel milled flat, or carefully shim the extrusions when bolting them to the steel.