Main Bearing Clearance

Undersize main bearings
340/318/273
http://www.ebay.com/itm/CLEVITE-MS5...73-318-340-MOPAR-DODGE-PLYMOUTH-/250900537402

360
http://www.summitracing.com/parts/SLP-4999MA1/

If you mean the journals too big...then u do need oversize bearings
Here are the rods..
http://www.cnc-motorsports.com/engi...triarmor-coated-bearings-0-001-clearance.html

When the term undersized is used..it refers to the journal diameter being undersize, therefore the bearing would need to be bigger to take up the space...if the journal is too big...then u need a larger inner bearing diameter=oversize