I know I am a little late to the game here but I thought I would mention a few things. Line boring these blocks is not really anything you want to do unless you absolutely have to. Every main is a different size and the machining process changes the orientation of the cam and crank gear, as well as the neoprene rear seal. So if you are going to go this route, you must make it clear that the absolute minimum amount of material possible is to be removed from the caps before the boring, yes boring not line honing.
Me, I would probably weld up/braze the broken lower part and skip the expensive line boring.
The issue of the screws coming in from the pan side to secure the timing cover was a left over from the main shim era. With the big trucks, after a lot of miles, you pulled the truck over the work pit, pulled the pan, removed the two screws from the front main cap, and from there you could remove the bearing cap and adjust the front bearing. Or, you did an in chassis rebuild, including the adjustment of the main bearings. Otherwise, if the screws were threaded to the main cap from the front of the cover, you would have to tear the radiator out, remove the screws from the front, which you may or may not be able to do with the balancer installed, and then pull the front main cap to adjust that particular main bearing.
After 56, the bearings were precision bearings, no shims, and so no adjustments. GM just opted not to redesign the timing cover, plate and front cap.
