You should be made aware that angular contact bearings require a mechanical method of applying and maintaining a preload, usually with threaded retaining nuts. Even if you buy 2 matched, preloaded pairs, each pair must have either the inner races or the outer races locked together. That is, a simple snap retaining ring "sitting in a groove nearby" is not going to do the job. Depending on how the original spindle was constructed, you might have to modify something to achieve this type of construction.
That high speed is going to generate some heat, too. The bearings really need to be lubed and cooled with an oil mist or spray system. If they run in grease, I suspect the drag will be too heavy, and the grease will be mostly thrown off, but the heat has nowhere to go. If the bearings are submerged in oil, the constant stirring of the oil will make heat, too.
If you decide to go ahead anyways, I think that you will not want to preload the bearings against one another across a 3" distance. The heat of the spindle will cause significant growth in length, and the preload will be altered. The more length between bearings, the worse this effect will be. This is why typical spindles have the bearings mounted back to back in pairs. The preload is entirely between the two back to back bearings, and not across the top end and the bottom end of the spindle. In many cases, the top bearing just acts as a steady, and floats a little bit in the housing. It is a precision fit in the bore, but as the spindle heats up, it can push this bearing upwards without fouling against a shoulder.