well, then we can do: make it 7mm thicker
or at very least (when we can make miracles come true and cram all the needed chips into it somehow somewhere) you need to cut new hole into the metal of the battery bay, to move the mini-sim holder out of the way (will cost a few bucks when we at GolDeliCo do that for you. Probably only feasible with laser cutting, on a scale of a few hundred cases). See photo. [edit] I checked back with my metal and mech guys and been told laser cutting and even more likely CNC milling shouldn't be a huge problem neither for process not for expense.[/edit]
Or we kick out the main camera
or no GTM801 LTE-modem, only GTM601 3G (if that fits at all)

PS: Option801 comes with 2 SIM interfaces ;-) A (2nd) micro-SIM holder might fit into the place next to uSD holder, when we cut a hole into plastic and bin one speaker. Err nope, there's an antenna. So *maybe* a combi-holder for uSD and uSIM stacked somehow?
[edit] weird idea: we will probably need to increase the gap between kbd and screen half by at least 0.5mm anyway. This theoretically opens up some opportunity to place a SIM slot-in holder there, somewhere between volume rocker and NOKIA label[/edit]
[edit2] why are you so scared about micro-SIM? There are adapters that make a mini-SIM from a micro-SIM, only you mustn't use those adapters in slot-in holders. N900 has no slot-in holder[/edit2]