If by inseam, you mean pant size, I think you are fine. Why are you letting some minimum wage LBS employee try and convince you that your 40 years of experience is wrong. The geometry of bikes is totally different today. Someone who has only been selling bikes for 5 years probably has no idea how to fit a bike with classic geometry. I was trying out some new bikes recently and noticed this for myself. My bikes all are set up almost exactly the same, yet one is a 60, one is a 62 and one is a 24". The road bikes have about a 2" drop from saddle to bars.
I test road a Jamis Xenith in a size 58. This is supposed to be their race geometry, yet adjusted correctly, there was less than 1" drop. The same held true for a Madone. I would have had to remove all of the spacers to get it down to 2".
So for a new bike, the salesman may have been right.