cracked valve seats
 

how much damage will cracked exhaust valve seats cause and will the engine run.

How badly are they cracked? By no means is it a good thing, but I bet most of the truck this old have cracked seats and are still running. Small crack will get by, but they can't get too long.

crack valves
 

there not trough the seat the last 6 heads I've looked at has had them in the exhaust valves is this just bad luck?

they usually always do on heads without pressed in seats and run fine.