FWIW, and from my experience so far, I think the removable dash is over-rated. If you have to remove the dash, you probably have bigger issues, and removing the spider will gain you all the access you need and save you a ton of time (and aggravation) in the long run.
With that said, you can access most if not all of your electricals through all the gauge holes, switch plates, and underneath. Just do a good, reliable wiring job (add extra wires for future expansion..), think ahead, think worse case scenarios, and place your relays. etc., so they're easily accessible. Make your gauge holes just big enough so that you're able to press in your gauges and avoid using the screw on rings with the small gauges (tach and speedo OK). Due to the thickness of the fiberglass dash, you probably can't use the rings anyway...
This all saves you from butchering your dash to try and make it removable. Also take into account that on a hot day, that black dash sees a tremendous amount of heat. Leaving it one piece will minimize the distortion and warping you will most likely get after cutting it into pieces...
There's a way to secure the front of the dash securely by attaching (or bonding) floating nut plates behind the front dash lip and securing them with counter-sunk screws at the front of the spider. I used 5 AN countersunk 8-32 screws along the front radius of the spider (hidden when front bonnet in place). It draws the dash forward nice and tight! Drill through the spider into the dash first to mark your holes...