That just fits inside the stock rear end housing that is already in there, right, and uses the existing axles, etc?
It depends on the year RX7, some of them came with different spline counts on the axles. I believe the earlier models had a different spline count. The later year RX7-TT models fit in the B2600i rearends. But I know the '84 GSL model fit into my B2200. Do a search on here or google, there is some info out there. You have to extend the B-series axles to correct the end play. In my case, I was already narrowing the rearend so I had to cut down my axles and respline them. I had a machinist do the work so I don't know the details involved.