I cannot speak about replacing the main bearings but I had this same problem on a 3000 one time.
Turned out to be the oil pump had a cracked housing. The oil pan took 1 to many licks from stumps.
I would check the oil pump.
You can pull one of the main caps off, if the bottom half of the bearing is OK put it back on the leave the rest alone for now. Do not try to get the top half out, the wear will be on the bottom half of the main bearings anyway.