valve stem seal failure?

Just a quick question about valve stem seals. I have a 1967 mustang with a rebuilt 289 v4 that appears to have one or more bad valve stem seals. The engine has been professionally rebuilt and has seen very few miles since being done, however, the car does not get driven much. It actually hardly gets driven at all. There is a lot of snow in the winter here and I travel for work in the summer so, yeah, aggravating situation. In any case, the car is always garaged and I started it up during the extreme winter cold this year to move it and, long story short, it now puffs smoke on start up regardless of how long it is parked, over night or a month. It is acting like a bad valve stem seal but what confuses me is the lack of miles. I do not think the seal was installed improperly because it has been fine for several years. I guess my question is, can a valve stem seal fail from extended lack of use or a start up in extreme cold? I was just looking for some insight before I tear into it.

I know this post is a bit old but alas, the mustang went back into storage for the winter so I did not get to dig in to this issue. I can say that I found oil on the threads of a few spark plugs (not a valve cover gasket leak) so oil is getting into the chambers. Also, plugs were oil fouled. I was able to do some research on what the motor has for valve stem seals. The heads are World Windsor SR iron and originally they came with rubber umbrella type seals. The shop that built the motor machined the guides and installed teflon positive stop type seals. Not sure how long teflon lasts when in storage conditions but from what I understand, teflon is not the goto for street use. Not sure why they went this route. Going to replace them this spring, any suggestions on what to use would be outstanding.