Ok has requested.

First 60mm box section will be more than enough so I'd save some money here. I'd also simplify material list and use it for the bed supports.

The gantry looks fine to me and wouldn't take Irvings suggestion of lifting/widening putting the gantry between the rails has the alignment and adjustment would be a night mare. Plus the bearings would be under less than Ideal load on there sides.

Would take Jons advice and move the spindle mounts.

The single motor or twin is mostly a personal thing and dependent on other factors like what your cutting and speeds required.
Has you know Personally I prefer the single and belt setup has I don't like having the gantry run out of sync causing racking potential.
Missing steps from high acceleration hisn't an issue because often to enable the high acceleration you have to sacrifice velocity, put it this way I don't have any and my gantrys quite heavy.
Infact missing steps becomes more of an issue with slaved motors because often folks try to run too fast and the margin between doing so and not is close. Plus motors can sneakly drop steps here and there at higher speeds has the torque drops away resulting in slowly running out of position one one side or the other. On short to medium jobs this can go unnoticed and ounce each screw is separately homed (if correctly aligned) the error gets cancelled out. . . But on very long jobs it does show in positional error and if too bad can even lead to stalling motors has the error builds resulting in racking the gantry.!! . . Theres nothing more annoying than stalling 10hrs into a 11hr job.!!

Obviously if motors are tuned at a safe level then it's not an issue at all. With single motor/belt setup positional error will still happen if over tuned but the racking and stalling won't so the job will finish fine.!. . . It'll just be wrong in some way.!!!
The single motor will also allow slightly higher feeds has you can tune it closer to the limit and doesn't need quite has big a safety margin.!

Like I say it's personal thing but both work fine if setup correctly it's just the single setup tolerates more error without doing damage.!

I notice on the Spec you have 1605 on Y but 1610 on X.? Whys that do you plan to gear 1610 2:1 to bring both into similair speed ranges.? Thou it doesn't matter each Axis working at different speeds Personally I like to keep the X & Y axis balanced speed wise then one hisn't slowing the other down or limiting the potential feed rates. ( IE you have Y capable of 5mtr/min and X 10mtr/min then set a cutting feed rate of 6mtr/min the X Axis and feed rate will be restricted to the speed of the slowest axis when doing combined moves, this will increase cycle times quite a bit.)