7th lord in 8th house is not a good position for the 7th house. As the 7th house is the house of marriage and also the spouse, the 8th house becomes the family of your spouse. When the 7th lord goes to the 8th house, it means that the inclination of the spouse will be too much towards his /her family.
8th house per say in a horoscope is a malefic position and therefore any planet when posited in the 8th house loses its strength. Matching of the horoscope becomes very important for marital happiness.