I didn't use them on my roadster and did on my sedan delivery. I haven't had the sedan delivery running and on the road yet so I can't really tell you how much difference it makes. I think most folks will recommend including them and unless there is some overriding reason not to incorporate them into your design, I would use them.
BTW, if I understand your mounting method (the lower rail of the body skeleton sits right on top of the frame rail), you might want to consider using 1/4" - 1/2" rubber continuously under the entire length of your body skeleton - or wherever it comes in contact with your frame. This would give you very even support all the way around. I know on my roadster I had a slight bit of body flex when it was lowered onto the mounting points and the door jambs actually closed up about 1/8" - which created a rub when the doors were installed. (This is a potential problem with almost any roadster design since there is no support at the top of the door opening.) I had to jack up the body directly under the door jamb and weld in a new body mount at that position to remedy the situation. If you run a continuous hard rubber cushion between your body and your frame, you can hopefully avoid any problems like that.
BTW, I found a hard rubber material for this use at my local ACE hardware. It is 12" wide and sold by the foot off a roll. It cuts fairly well with a straightedge and a sharp exacto type knife. It's about 1/4" thick but you can glue and build up layers to suit your needs.
Always learning...and sharing what I've learned.
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