How about placing the bulkheads on the bottom of the external overflow box? This would shave off a bit from the total footprint (front to back). Provided you don't glue the standpipes into the bulkheads it will be simple to tweak the heights of the elbows as you can just cut pipe to the right height.
A longer weir will make less noise than a shorter one (assuming the same amount of water travelling across it) so I think wider is better, assuming it will not get in the way of other equipment (such as light stands / return lines etc).
Pick a pipe size (25mm?) for the drains, then look for bulkheads (often called
tank fittings) to fit that pipe size.
See here for the hole sizing chart for hansen fittings.
3. Where to get my tank cut?
Drilling the holes for the bulkheads should be an easy DIY job (Google will find you loads of tutorials), or you can ask around LFS.
In your drawing it looks as though the weir is ground out of the back pane of the display tank. You could try glaziers or LFS to do this, but it takes a lot longer than drilling holes so costs more in labour.
My first tank was built with an integrated weir and I doubt that I would do it again - an alternative is to have a second, internal box for the weir (such as black acrylic) and then drill the rear of the tank to allow water to flow from the weir to the rear overflow box. The internal box can be quite slim and unobtrusive because it does not contain any plumbing. You also have the option of having a much larger weir (eg. internal coast to coast) with a smaller rear box.
It could be a good idea to plan where your return line will go before committing to anything.
