I recently purchaced a (very) low mile 68 sedan w/ slant 6. It has stock
13" rims. The spare still has the Goodyear 2 ply bias tire!
Can I get 14" steel rims for this car? I am not looking for performance
tires or racing ability. I just want a little more meat on the street.

Yes, as long as you maintain the total height that ws designed for the car it doesn't matter what size wheel. I believe around 24-25" total is your height you need. A 205/70/14 is approx that size & will not change your speedo much. You can put taller/shorter combo's it just changes your original speedo accuracy. I had that size on my 69 dart with no problems.

There is a very good tire size site: http://www.miata.net/garage/tirecalc.html if you need to determine height and width from one size to another.