As for the seal, reuse it if it is not damaged. It don't do much anyway.
I have bearing numbers off of timken boxes for stock bearings and off of koyo boxes for angular contact bearings.
timken 30207M 9\KM1 (lower)
timken 30206M 9\KM1 (upper)
koyo 7207BGC3 (lower angular contact)
koyo 7206BGC3 (upper angular contact)
The only real advantage of the AC bearings would be if you intend to run the spindle rpm over 6,000 rpm. Otherwise it is best to stay with the stock bearings.
I run AC's in my spindle for 6120rpm with the bottom in oil bath. I fitted seal to bottom and plugged the tang slot of quil with aluminum plug and some JB weld.
Even the stock bearings are rated to 6000rpm range with proper grease, use of wrong grease will allow it to melt out of bearing and leave it dry, or, it can be vaporized under load and leave behind a dry paste which gets displaced to side and bearings run dry. This is why the quil \ spindle should be on a yearly maintenance schedule. I can re grease and oil my spindle in about an hour, leaving little excuse for not doing it regular. My first set of bearings smoked and spun within the first few months of use. That is when I found that they were contaminated with iron fine and that there was very little grease residue left behind implying insufficient grease. There is lots of room in there so pack the bearings fully and ridge them on outside with some more grease.
I forgot to mention that a cheap grease seal for the bottom of the quil\spindle is made from the tongue of an old boot. Use the threaded retainer as pattern and cut the leather slightly larger for od and slightly smaller for id and put it in with the threaded retainer. This will keep the grease from running out when it melts and it will last for several years at least.
Don
IH v-3 early model owner