I quit using those special valve stem seals 30+ years ago and I abandoned umbrella seals about 20 years ago when I discovered they were no longer made in the USA and they would crack, fall off and make a real mess (which happened in my engine). And once those umbrella seals are gone (assuming you didn't use the O ring type), you get lots of oil running down the valve stem. I've only used the flat O ring-like seals since then and have had no problems. Like Jerry said, you can over-seal them. The bronze liners should be good and if I was to re-do a head today I'd look into those, but if your valves and guides are done correctly and to OEM specs the original style should also be okay. Some people might start throwing rotten tomatoes at me but you can also have your guides knurled and that will help with lubrication of the guide. If you find somebody who knows what they're doing, this isn't a bad choice, but today finding that person may not be easy. It doesn't sound like overheating is a problem. Sorry to hear of your trouble. Not sure what to say about your machine shop.
