I have logged two tulip poplar cruck trees on my lot. They have an inside diameter of more than 25' of length. I am now planning to leave them round for strength (18" & 15" crosshairs at the butt tapering to 11" & 9" at the tip). I am eager to hear any and all interest and advice but particularly wondering if a kerf cut trunk to tip on the inside to the heartwood might reduce later checking. I am already end sealing and spraying peeled surfaces with brine.

I'm assuming that you mean 25 feet apart at the butts. The carrying capacity of the crucks will depend on what's coming in from either side. (What is the "bay" spacing.) Also, what are the required design conditions in your area?

As long as you make the kerf cuts in the plane of the cruck, you should be OK. However, you probably don't need to kerf more than about 2" deep. That much kerf will should be enough to cause any cracks to start at the kerf.