Do you have a REAL race seat with the opeinings and have you installed a real 5-point belt-harness setup. If so, then you are part way there. The next step is a solid horizontal bar behind the seat at about shoulder height-take a look at the Simpson catalog to see the right way.

A tower bar will not do what you want. The alternate would be to bolt the harness to the floor but that has issues as well. The important thing is strength and location.