My only fatbike has room for 3.5" tires at most (it is older than the fatbike phenomenon, so I designed it for 3.0 Nokian tires plus mud). I used Snow Cat rims, which are 45mm wide-- like the tires, the widest that were available at the turn of the millennium.
If I were building a fat e-bike today, I'd skip over the expensive, relatively lightweight Surly rims in favor of Weinmann DHL65, DHL80, or DHL100 rims. They all come in 36 hole drilling, they are all double-walled, and they retail for about $60. That's a deal. Just about any bike shop can special order them.
I'd use 14/15ga Sapim spokes from danscomp.com, because at $0.40 those are by far the the best deal around, and they will cut them to any length you like. Hub motors usually require some weird length. Go for cross-1 lacing. Don't let any bike shop guys tell you different, because they've probably never laced a giant diameter hub and had to figure it out for themselves.
I'm not fully up to date on the hubs these days. What I know from running hub motors on 700c and 29er wheels in the past is that you want a slow wind motor. If you get a fast wind, like something that promises 25mph or more at 36 volts, you'll bog it down with a big tall tire and get terrible efficiency. You can always add voltage to get higher speed (within reason), but you can't lower a motor's RPM sweet spot if it's too high.
For instance, the simulator at ebikes.ca says that the Nine Continent 2808 motor (the slowest wind listed there) will make over 1kW of power from 14 to 18mph with a 29" tall wheel, 48V battery, and 40A controller. It will top out on the flat at about 25mph, at 81% efficiency. It will climb an 8% grade at 16mph at 60% efficiency (which will overheat it in 5 minutes). These are good realistic performance benchmarks. You can use a faster wind and get a higher top speed, but you'll have more power consumption, slightly lower efficiency, and slightly faster overheating.
Something between few and none of the fatbike tires currently available are going to behave nicely at 30+mph on pavement. If you must go that fast, you do what you have to do-- but if you are content with below-30mph top speeds even with help from the pedals, you'll yield better range and efficiency that higher speeds would give you. And you'll lower the climbing speed at which the motor really bogs down and cooks itself.
I don't know what motors come with axles long enough to be adapted to your Mukluk's 170mm rear dropout spacing. Maybe others here can point you in the right direction.