I think the reason for bushing the bores has been missed. The main reason is oil control. A roller lifter requires far less oil than a hydraulic lifter so bushing a stock block reduced the flow and pressure loss around the lifters and sent that oil where it was needed most. Generally a bushed lifter got a very small oil hole drill into the oil galey to ensure some lubrication to the lifter. The "bronze" itself provides less friction than the cast iron so that's why they use bronze instead of just installing a steel sleeve.

Using a larger diameter lifter might be a reason in today's modern blocks but not "way back when" in stock blocks. There just wasn't enough material in that area to hog the bore out and bush it and have any strength left in the area.

Lifter alignment ... that may have been a bonus but I seriously doubt there be any performance gain - just how far out could they be ?!?!