Pete---I don't want to be an alarmist here but a few things to look for as you take it apart--
That main support may actually be distortong the bottom of the block---
The block could be split from the camshaft bore to the main bore along the
oil passage hole---
If this is a 289, you might want to seriously look at going to a Dart SHP 4 bolt block at 4.125 bore---