Trying to attach my old steering wheel to my new hyd helm with the standard 3/4" tapered/keyed shaft..
The spacing between the 3 steering wheel mounting bolts is 1.5"..
I have found some 3 hole adapters but they all have 1.75" spacing.
Does anybody make the adapter I need?

After a bit of digging, I've discovered there are 2 ways to measure the distance for the 3 steering wheel bolts:

Method 1: Measure from the center of one bolt hole to the center of another bolt hole (this is what I did).

Method 2: Is the diameter of an imaginary circle centered on the steering shaft. The circle cuts thru the center of the 3 bolt holes.

On my steering wheel, the imaginary circle diameter is 1 3/4".
From bolt hole to bolt hole, the distance is just a hair over 1 1/2".

With this in mind, it appears the adapter is not so rare after all.