This is my opinion on sealed bearings for the old 3 speed transmissions.
For the front ball bearing, it's not necessary to use a sealed type, particularly if the trans uses the 1937-48 front bearing retainer (which houses a pressed in lip seal). If your bearing retainer is the 32-36 type, or the 49 and newer retainer, it won't hurt to use a sealed ball bearing. I recommend prying the plastic seal out of the inner side so that gear oil can always lube the bearing. We just install the 78-7050 retainer with the lip seal.
The rear ball bearing is a different matter. For several reasons I don't recommend using a sealed bearing. On closed drive transmissions, it's my contention that Ford intended for some gear oil to get past the oil slinger, and migrate through the ball bearing to mix with the grease in the u-joint area. This cavity is sealed with the paper gaskets and cork strip. If properly sealed up this area should not allow any leaks of significance.
On open drive transmissions (i.e.: pickup trucks) I believe the rear bearing should also be the open type. A bit of gear oil is necessary to lubricate the output shaft splines (in the slip yoke). Any excess oil would be retained by the rear oil seal in the rear bearing retainer housing.
Rear leaks are more common if the trans gearbox case is not sealed up well. We've found that the biggest source of leaks here is the failure to seal the passage for the countershaft lock pin. This is open to the inside of the case. Remove the pin, clean the passage of oil, clean the pin, and use a good sealer on the pin, and over the ends of the pin after reinstalling.
Stay away from synthetic gear oils....they tend to find tiny crevices easily. Watch for driveline slope on lowered hot rod applications. The rear end oil can run right down the torque tube and overfill the transmission.
These are just my opinions and are subject to change with my next whim.