You're frame is broadly the same design as mine and even partially built I can tell mine will be strong enough so you shouldn't have any problems.

In terms of strength I'd drop it down to 50x50x4 to save some cost. If you look around here you'll find a spreadsheet that does gantry deflection calculations. The span on the the x-rails (between leg and centre support) is the same sort of problem, if you plug in the details of your steel you'll find that the deflection is tiny (<1um I'd guess) and it'll also be tiny for 50x50x4. If anything that spreadsheet will over estimate too since the ends of the rail will be welded into place.

The only thing I'd say about 50x50x4 is that is doesn't give you a lot of room to mount the rails on. I wish I'd gone for 60x60x4 now but that's the great thing about hindsight.

From someone that has tried to bolt together his machine to someone about to try, don't bother. It's very slow and you don't get any meaningful adjustment at the end. In SketchUp it's really easy to place a bolt hole and give yourself a perfect 1mm of play so you have some adjustment. In real life you won't be able to work to the sort of accuracy necessary unless you have a full on machine shop in which case you'd be better of just buying a CNC. Like you I'm going to bolt the main x-rail on but pretty much everything else will now be welded together (I've got the welder I'm now just waiting for a workshop that has sufficient power). I'm not entirely convinced that bolting on the x-axis rail will help all that much in preventing distortion, I've tried welding some of my 50x50x4 and I couldn't spot any distortion. What bolting will help with is shimming if you find your two rails aren't quite parallel in the z dimension.