What size front tires?

[bm82]

I have 15x10's on the rear with 255/70/15's and the front has 15x8's with 235/75/15's. The front look to be just slightly taller. What height/profile tire can I step down to in the front and still be safe without overloading the tires? I was thinking 225/70/15's but finding a used set is almost impossible as it seems to be an uncommon size. With the front's being 8's I worry about going with a 215 as it just seems too narrow. For now I'm going to stick with used tires due to the fact that I will eventually change wheels once I get it finished and I'm not going to be DDing it for a while. This is on a 1968 F-100 shortbed. Once I get some smaller tires I will be taking one coil out of the front for a little more stance. It won't ever haul any heavy loads or tow anything so I'm only worried about carrying the factory load rating.