Ok....so small bolt pattern vs big bolt pattern infos !

Hi guys , my 1969 valiant 100 was born with small bolt patern 4" , it must be exactly the same for a 70 duster, i am better off buying the front and rear suspension ( differential etc ) of a 73-76 duster or work with what i have already and swap good disc brakes on a 4" bolt pattern , my car was born with a 8 3/4 and its still on it