It is only my opinion but I believe they are cheaper, stronger and quicker to produce than the aluminum pieces. I.e. one operation to cut the shape and holes via laser and then bent 90 degrees as opposed to probably a bunch of machining to an aluminum block.
I don't think there is any difference in where the bolts go to mount the upright to the a-arms so no replacements necessary.
I.e. aluminum pieces probably look better but steel pieces are much cheaper to produce.
Which reminds me of a friend who many years ago was building a street rod. I was visiting when he was polishing an aluminum piece to a mirror shine. After he was done polishing it, he assembled it into the front end of his street rod. You couldn't even see it once it was installed. I asked him why go thru all the trouble for a piece that wasn't visible and his response was, "I knew it was there!"
I don't think there is any difference in where the bolts go to mount the upright to the a-arms so no replacements necessary.
I.e. aluminum pieces probably look better but steel pieces are much cheaper to produce.
Which reminds me of a friend who many years ago was building a street rod. I was visiting when he was polishing an aluminum piece to a mirror shine. After he was done polishing it, he assembled it into the front end of his street rod. You couldn't even see it once it was installed. I asked him why go thru all the trouble for a piece that wasn't visible and his response was, "I knew it was there!"