First time build - Steel Frame CNC Router

[examorph]

Noticed that I might have some difficulty cutting the motor mount slot accurately on the gantry side, so made it a separate adjustable support plate allowing for the Y axis ball screw to sit more central, more adjustment and lets me get started on welding the gantry before getting access to a mill

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I am going to start welding soon and have next to no experience with this so hope you guys can keep me right:

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1. Whats the best way of handeling this, tack up the full frame whilst checking squareness and then weld up fully when happy or do it all in sections?
2. I read on here that short runs are the way to go. Would a 50mm single run along one side of the 50x50 box section be ok or do I do this in two separate runs (25mm)?
3. Are there any parts of the frame I could stitch weld? Maybe the x axis rail support and all of the gantry/side plates? If so, whats the typical size of weld and gap between stitches?
4. Any tips on this/recommended welder settings?