Multiple stretch distances need minimum/maximum dimensions

[RapidCAD]

Hi all, I'm trying to find a way to build some rules into a dynamic block so that the machine it represents can be ordered correctly.
The attached block is for a conveyor where the overall length can be ordered from 120" down to 70", and the edges of the rectangular tubs within can not be placed closer than 5.5" from either end (in any configuration). Additionally, the two tubs have to keep a minimum separation of 3".

I've been trying to come up with a way to place a linear parameter or constraint between the tubs to limit the minimum distance to 3", but so far I can't find a way to do this. Does anyone have an idea or understand those pesky constraints better than I do?

Any help greatly appreciated.

Ron