My '37 Plymouth still has the original sheetmetal covers on the rear springs. I've never bothered to try greasing anything but the shackles and have had no problems with squeaks. I've seen late '30's Chevies with sheetmetal covers also. I have something I think is called Polyride that is a thin plastic the width of the spring that you put between each spring leaf. You can ge it from most streetrod parts suppliers. I have been using this on my '53 Chevy for 10 years now and can't say that I have had any problems with it other than it trying to squirm out at the ends once in a while. Not sure if it really made any difference in the end.
If you are trying to do a "points" type resto, then you should get in contact with the group that does the judging and see what they use for criteria and follow that.
