Here is some service literature from Ford in the mid-seventies.....ignore the vacuum part. 
The 25° you mention - and the distirbutor rpm (half crankshaft) reported, appear much like the upper end of the curve shown. That curve is from a passenger car version of the Motorcraft distributor with a light spring (kicks up steeply at the low end) and finishes with the heavier spring that tapers off considerably. This was probably used in the so-called Windsor, Cleveland and 385 series motors.
The Y engine will actually work well with a 35°/36° advance top limit - that being the sum of the initial set point at idle and the total mechanical in the distributor. So I'd suggest an initial move of "power timing" the engine by setting the timing at a full advance and letting it idle back to show you the initial required to get that.
Replacing the springs is the method by which you may change the ramp rate. On distributors like the one plotted here, that means replacing the heavier spring with one like the lighter of the two. Maybe get all of the 25° to be in by 3000 engine rpm. The additional advance will lean out the system - and probably require re-tuning the carb.