Filter Return drop w/ Vanes.

So getting my new furnace installed shortly. They are redoing the return drop. My previous furnace had a 2400 space guard mated to the side inlet and then an 8"x29" return duct drop mated to the side. No vanes or anything. New filter will be Aprilaire 2410 for the Amana 80K BTU Variable Speed furnace. Was told the Amana says 16x25 filter size and was told opening was 14x23.5. Both the Aprilare size and furnace size are about the same so that is a plus over previous. I asked installer for vanes since it will probably be a rectangular duct again. Since the new furnace is narrower and the new filter is narrower then the older space guard there is some additional space between the drop and the furnace. I want to make sure they do this transition properly.

What I mean is, the old drop was 29", which mated fine with the filter cabinet depth. But. since the inlet in the filter box is narrower should then the drop be the size of actual opening of the filter cabinet itself vs. the overall width of the filter cabinet? Then some type of transition to bring that to the overall width of the filter cabinet box? Otherwise, if the drop stays at like 29" W, which still would mate with new filter cabinet I believe and the vanes run 29" wide but the inlet on the filter is only 23.88" W, wouldn't there be resistance or loss since the filter opening is like 5" narrower and air is hitting the side of the filter cabinet?

source https://hvac-talk.com/vbb/showthread.php?2222186-Filter-Return-drop-w-Vanes&goto=newpost