Convertor bolts should be audible when they hit the block. They will not be in contact with the block at all times, meaning you should 'feel' each of the 4 bolts catching the block edge.

The engine turns hard at all times so I think the problem is elsewhere.
I think I would want to pull a few bearing caps and have a look.

If you can get the oilpan out in the car, I would probably start with checking the mains, after you inspected the oil of course.

Maybe even quicker, take off and cut open the oilfilter and have a peek inside.


edit:
Just reread everything and it got clear this rotating assembly has been running fine for 45 miles in your previous, '65 engine block.
You just changed the engine block and the problems started.

Does the '68 block have the exact same cylinder bore diameters as the '65 block?
I'm now thinking of pistons maybe starting to skuff on the cylinder walls.