What's up all. I've heard that it was a bad idea to install new/rebuilt heads on a high-mileage motor. Is that true,and if so, why? Is there anything I should watch out for? Thanks for any replies.

I say it has more to with large amounts of cylinder psi increase.
this will accelerate the oil being pushed past the rings=smoke.

do a leak down test if it worries you.
anything beyond 10-12% is bad.
5% is a good average.

I say if the leak comes out good & the vales are sunk or not sealing, go for it.

If the motor is in good shape changing the heads will not hurt a thing unless you are going from open chambered to closed heads. Then the higher comprerssion my cause a little oil gettiing by the rings. I wouldn't worry about it.