I attached a pretty basic sketch of a typical trailer. My boat is supported by the two bunks and roller #1 which is under the bow stem of my boat. In fact, my trailer does not have rollers #2 & #3.
In my opinion, rollers #2 & #3 are there to help center your boat and keep it from scraping the trailer when you load it. When your boat is loaded, again my opinion, #2 & #3 shouldn’t touch the keel. Why? Because unless you have them perfectly adjusted, the chance is one will be more in contact than the others. When your light aluminum boat bounces as you go down the road, a slightly higher roller could dig a divot in your hull.
My thinking is three points of contact best eliminate movement – kind of like a three-legged stool is less likely to rock if the legs are not perfect. Again, this is just my opinion. I acknowledge others feel that the keel should be supported along its length by multiple rollers. Whatever you decide, I think we would all agree that the keel should not be resting on the trailer frame.
Another important point is that the bunks should extend just past the transom and the transom should be fully supported by the bunks. If the bunks are too short, you risk deforming the hull because the bunks will be pressing only against the thin skin of the hull. Folks call this putting a hook in the hull. If the bunks are long enough to extend past the transom but leave any gap between them and the transom, the same thing can happen. I guess the best way to say it is the transom has more “structure” to support an aluminum boat.
Additionally, I think most of us would agree that the bunks should fall between bottom strakes.
Most manufacturers recommend a set up for their boats. And, I guess they should know. I realize your boat is a 1950’s model, but Mirrocraft might still be willing to suggest a certain set up if you contact them. I really don’t think it necessary and probably wouldn’t if it were my boat, but it is a potential resource if you want. Another thing you can do if you are not sure is check out some of the set-ups at your local ramp.
After your trailer set up perfectly, consider using side guides or side bunks. That way your boat will settle right down where it belongs every time, even when it is getting moved sideways by wind or current.
Once you know where you want the bunks/rollers, using shims to make the adjustment should be OK as long as everything is secure.
I am not the oracle of all trailer knowledge. Hopefully others will chime in. But I do hope this helps some.