I've got a couple of great stallions in my breeding barn that have won 14 and 12 group1's each. The first horse had a preferred race distance of 1m3f but won all his group1's at a mile so my question is how do i breed him? Does his preferred distance outway his race form at a mile?

I'd breed him with milers and sprinters personally.

You can usually tell if you've got a 1m3f horse that likes a mile because his stats will look like a miler (not too much stamina (about 1/4-1/2), plenty of speed/accel).

Preferred distance is only a rough guide, he is a 1 mile horse but don't confine him to breed with only 1m mares. Breed him with sprinters and horses that get up to 1m4f, the more offspring he produces the more chance of finding a horse just as good or even better than him.
Good luck.

i would breed him with 1m5f or 1m6f horse and you could have a derby winner