Racked up 400,000 k on my last dual fuel 4x4 , never had an issue with the heads on either motor I had in that 4x4.
all I did was alternate as you have suggested between gas/ulp.
How long do you intend keeping car???
Head would need refreshing after 250k anyway,
All I can really say even though I've had a good run is "gas kills engines". Put a valve kit on it if you like, but not better than running ULP