Not enough clearance. Crank grinders like to grind your crank on high side, bearing manufacturer likes to give you bearing close to max wall thickness and old blocks main bores shrink from heat cycling. Now that it has bearings that look like these guaranteed main bores have shrunk. Probably needs line honed.

When you get crank turned have turned .011-.021-.031 US to give you more bearing clearance. Clevite now makes bearing in std with ex clearance so if you have a new or good std. crank use part # MS1266HGX

An engine with a little to much clearance will run forever, one a little to tight will die young.