The 1972 Chrysler has a very inefficient lower unit. The props are decent but not the best hydrodynamically. The engine was rated at the crankshaft so it is really only delivering about 55-60 HP to the water. Your 15 foot Elgin is probably a round bottom boat. It probably sits a bit low in the water in the stern from the engine weight. All this is not conducive to higher top speeds.
Without knowing the wide open throttle RPM it is almost impossible to recommend a different prop. However if you are a gambling man you can try a 17 pitch prop--if you can find one.
Now, just for a rough comparison, my friend had a 75 on a 16 foot Chrysler boat and he topped out at 27. I had an 85 on a 14 foot flat bottom boat and it reached 38 with me alone. Put two or more people in it and speed dropped to mid to high 20s..Smaller boats are very sensitive to load.
One thing you can do is to be certain the anti-ventilation plate is at least even with the hull bottom. These engines had a leg one inch longer than most transoms and usually can be raised at least one inch.
