Steve, welcome aboard.
I think you just purchased a Cox driver from me on eBay and I thank you for that.
I build lots of hardbody cars and use various mounting procedures.
Here is a simple method I often use. Here is a build I just completed using a Monogram '37 Ford panel.
I use the Plastruct product shown here. A package should last you years. Sometimes I tap it for a screw as shown here, but usually I sink an insert in with a soldering iron as Gary suggested. I make gussets as shown using .040" Evergreen sheet. Here I used one but you can use more if the post is longer. I crazy glue it in while it is screwed to the chassis to get the wheels centered in the wells. A pet peeve of mine. After it dries a little I go around the glued area again with some more glue for strength.
Happy to answer any questions if you have them.
PS: Save those Scarab posts for the Monogram reissue kits that have no posts. You can use them to make these bodies drop right on to Monogram chassis.
Joe