[Cal Metal]

Blykins:

One of the signs that the bellhousing hasn't been aligned properly will be the grinding in reverse when shifted to that position at idle.

Essentially, the pilot of the input shaft is cocked in the flywheel and is not freewheeling. That, in turn, will do a major job on your input bearing over time, and probably take it out, as well as the cluster gear if you are not careful.