Thanks to all for this input. Very much appreciated. Jbarca, great to hear your situation is solved. I looked very briefly at 16" wheels, but with the old 5 x 4.5 bolt pattern I didn't see any right off. They're probably available, but I just didn't see any in my quick look.
And its great to know that others with similar type wheels have found solutions! I haven't yet seen the inside of my rim, without tire on, to see how close the hole is to the rounded areas that caused Jbarca so much grief.
With the valve stem in the wheel and tire mounted I couldn't be sure of the stem hole size so this morning I emailed the Series06 folks and again they were very responsive to my email. They confirmed that the stem hole size in my wheel is 0.453" diameter.
So with the hole size confirmed, it seems like a good first step would be to order one of the Haltec N-1600 valve stems to see how it goes with one wheel (thanks mrekim for that product and measurements). I checked their website and that stem is the 0.453" size and all the other measurements seem to be inline with what would work well. It is listed as an aluminum stem which I hope is strong enough.
BenK, I checked the Myres catalog (thanks for the link) looking for something like the Haltec N-1600 in steel rather than aluminum but I couldn't find anything. If you know of something please let me know.
I'll order the Haltec stem and report back.
Many thanks again to all,
Marty