First, thanks for all the info. I’m not sure I’ve figured out a solution from all the variables you’ve mentioned, but at least I know now that it’s a fairly complicated question.
We resize the slides before putting them into the project, and we’ve tried various ways of doing that. They resize without much visible distortion, but that may be aggravated when Premiere gets a hold of them. We’ve also tried pillar boxing the slides when they’re in the timeline by leaving them strictly 4:3 square pixels but that doesn’t seem to make much difference.