[chrisguyw]

Hi Anthony, It is certainly your choice on whether to paint on the inside or out,........both have merits, on the inside, protects the decals (assuming that they are also stuck on the inside), and does produce a nice shiny smooth surface, if that is what you are after.  On some cars, that nice glossy surface does not really look particularly authentic (especially on older model cars), and I do paint many on the outside. 

As far a body mounting, I personally prefer "pin" mounting, as it is "period correct", and, is far less obvious than screw heads or clips........you can paint the pin heads body colour, and they do tend to disappear. It is dead easy to modify your Dynamic mounts to accept pins.

From a pure modelling perspective, I do understand your desire to eliminate any surface irregularities (pins/clips/screw heads) and for these types of bodies, there are few other options other than your Velcro method.

On any chassis with pans level with the bottom edge of the body sills, you can use some clear tape along the pan, and up the side of the body,.....while this does work quite well, and does not look horrible, the tape does have a tendency to crack over time/use, and is a bit of a pain to remove/re attach the body.

You can glue some thin sponge rubber strips to both the inside of the body and to the outside of your Dynamic mounts, and use screws/nuts, through the holes on the front of the Dynamic motor mount to secure the body mount brackets.. This method is used fairly extensively by the 1/24 IMCA guys to secure the "Mecanno" type chassis to their hard bodies.

This has proved to be a secure/strong method, and does provide some cushioning for those invariable "offs".

Hope this helps, but, I gather you already knew all of this.

Cheers
Chris Walker