You are stressing needlessly, as others are telling you. The front fork spacing will likely be 100mm - just about any bicycle wheel will fit. And the front end is the easy part. Unless you intend to cruise 35+mph for hours, just about any wheel with 36 spokes laced 3 cross will do just fine. If it is a 26" mountain bike wheel, just about any reasonable sized clincher tire will work from, say, 1.5" to 2.25".
The rear wheel is where you will find problems but not due to spacing. Generic 26" mountain bike sized rims cannot handle the forces over the long term.
I don't think I will be cruising 35+ mph for hours. I really don't plan on going much over 25 - 30 mph in the beginning. My main concern is here in Houston, the roads are all horrible and I want something over kill that wont deform if I hit a bump in the road moving along at 25 - 30 mph.
Aside from that the rear is where I am really having some problem with. I think instead of doing a multi speed build I will just go a single speed build with a single 44T cog for the drive and be done. I can buy a HD hub that has threaded adapters for the disc brake and for the drive cog mount so its not mounted to the spokes themselves. Not too expensive either. Just need to hit up the internet and do some reading on if there is any special matching you have to do between rim and hub.
This is the bike shop down the street from me that I would take my hub to and have assemble the wheel going to do some reading on them seems like they are good but some what pricy.
http://icycle-texas.shoplightspeed.com/
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