I think the preload is stabilized a bit by the fact that the spindle shaft is so long that it stretches a bit under high preload!
Both of the new bearings are slightly shorter than stock TRB's but the spindle nut will thread on further and almost needs a shim to set preload but should work with no mods.
I did mine and it was no problem and I found a brass cage USA bearing for $22 that gave .0002" runout.
I often end up milling deep profiles with larger cutters in steel like a 5/8" x 4" carbide cutter and I don't see the single bearing being a problem, machine flex and tool pullout yes, but not the bearing so far.
My preload ended up a bit high so I still have a fair amount of heat but no problems so far. Its making about half the heat of the TRB.
There is two small drilled holes in the lower spindle housing to help you drive out the lower race, they are not real easy to see if you don't look for them. The fit is tighter than really needs to be so clean the seat well or the new race wont seat. The upper race can be driven out by a long screw driver with a bent tip or just a shaft with a bend at the end.
If you don't go over 6500 RPM much the top bearing could be left in. It is smaller and will do better at high speed than much larger TRB at the big end.
I bought a spare 7207 Nachi to have sitting around just in case I need to do a quick replacement.
I think with some luck you could change out the lower bearing in around two hours.
I don't think there is enough metal in the housing for two bearings and considering how long a single should last and the replacement cost of a spare bearing, keeping a spare around might be just as well.
I did spend some time looking for a good low cost spindle grease as I have 4 that need re packing. I went with Mobil SHC100 NLGI 2 lithium synthetic.
It has a similar melt point and thickness as the Kubar NBU 15 and cost about $13 a 13oz tube. McMasters has it.
"The official grease of NASCAR."
So far so good on the IH A/C spindle bearings.