66 Dart

Breaking the motor loose depends on why its locked up. If it spun a bearing last time it ran it may not free up without taking it apart. If it truely last ran in 75 thats 40 years + so i would guess the rings rusted in cyls. If thats the case youve done the best thing by putting oil in cyls and letting it soak. Ive had luck by going to flywheel and prying against teeth to try turning motors over but starter must be removed for that method. Again you may need to remove and tear apart. Best of luck.
Rod