Shoulder straps should be mounted at a similar height to the tops of your shoulders (within 15degs or so), usually on a harness bar integrated into a cage. Lots of people do attach them to the rear seatbelt buckle mounts though, but how safe it is in reality I really don't know. It is possible that in an accident it could compress your spine due to the angle.
What about if i mounted them along with the rear belts, using the same bolt as the rear belts i mean, im not using the back seats but i dnt really wanna take them out!