Camaro77

We deal with problems like yours quite often and most times it comes from the customer buying something online at a better price than the prices available from an Dart dealer---often times, someone will buy heads bare and then put in cheaper components to complete a package, valves,seals,springs, retainers,etc

Most likely you have gotten some heads that have undersized stems(too much clearance) or more likely the wrong seals( a .531 seal on a .500 guide) and the seal won't stay on the guide boss.

If you have the capability to use air and remove the springs, release the air, and see if the stems have a lot of wiggle---
Measure the guide top for sizing and use the correct positive seal---

If the stems have a lot of wiggle---remove the heads and return them
If it is the wrong seals, just change them on the engine without removing the heads

As a note---We have seen head packages bought on line that had .006 -.010 guide clearance and they did much of the same thing that you are describing

There seems to be many valve stem sizes out there today that vary several thousands( maybe inch/metric conversions?)


Jerry