I stuck my neck out on a simmiliar question in the Boss forum about a month ago and no one corrected me so here goes again: (I am by no means an expert and I'm not claiming to know this info 100%)
We use just plain 1 1/4" RUBBER valve stems with the "DILL" brand caps (with the crown) on them on our '65-'66s, many Mustang parts suppliers have sets readily available but you CAN get the 1 1/4" valves at most tire shops still, and if you look in their take off caps pile you can usually find plenty of the DILL crowned caps, these DILL caps even came from the factory on our '06 PT Cruiser!
I really don't think that any type of Chrome valve stem came from the factory on a Mustang wheel from the first generation anyway.
You might want to ask Jack at Dead Nuts On, I think he is very knoledgeable on the '69-'70s
www.deadnutson.com
