When I had first read this article I took it with a grain of salt. http://www.camaros.org/bellhousings.shtml
Being I had the motor out of the car during its recent restoration I decided to buy a go/no-go fixture like mentioned in the above article (found one on eBay) and put it on the crank and bolted on the bell housing and I was surprised the test ring jammed solid when trying to go into the transmission opening. I thought maybe the fixture was defective so I bought a magnetic stick on electronic dial indicator from Harbor Freight. Boy was I surprised how out of alignment the bell housing was in respect to the crank. It measured .011" total run-out and the limit was .005". The deck squareness was also out at the 7 o'clock position by a few thousands. So now I had two test tools that indicated I indeed had an alignment problem.
I was able to reduce the run-out down to .002" max by using Lakewood offset dowel pins in place of the factory pins. After determining which direction to shift the bell housing, I finally was able to dial-in the pin positions before locking them down. Afterwards I had to place a flat shim between the bell housing and the block by the lowest left side bell housing bolt to get deck squareness to 0.0" . After all the alignments were done and re-checked with the dial indicator and the go/no-go ring slid through the transmission opening smoothly, and a new pilot busing installed, the transmissions slid into place with absolutely no effort (after aligning the splines of course). Before I always had to jiggle the transmission around a lot
to finally get it to seat into the bell housing opening. I don't know if the cause of misalignment were due to factory tolerances or block core shift from age or a factor of both.
In the past I was always going through bronze (real bronze) pilot bushing and even a needle bearing pilot bushing, and once an input shaft bearing on an otherwise rebuilt transmission, in such a short time of driving. And it always would grind slightly when going into reverse. I had to use the forward then reverse shift trick first. I could never figure it out until I read about bell housing alignments here at CRG. I found articles and how-to's on You Tube to check and adjust the alignment and it is really easy once you understand how it's done.
This is why I will now always suspect if the transmission is grinding in reverse and it is not attributed to other obvious mechanical issues, that the cause can be due to interference friction drag on the input shaft machined nose being wedged against the pilot bushing wall due to bell housing to crank misalignment and not allowing the shaft to completely stop rotating when depressing the clutch.