dimensions of a sunroof assembly on a 126

I am having the sun roof assm. replced on my 90 300 sel. the mechanic says the replacement is a few inches shorter any won't matchup. I checked with the supplier and was told it was pulled from a 91 3000 sel. Any ideas on why there is a difference. How can i make sue i get the right size? I was also told that all 126 models from 86-91 carry the same assm. please aasist