If you feel brave enough to make the initial cuts through the fence I have illustrated below, that should be the scariest part of the operation.
Actually the hole in the auxiliary fence can be cut with a jig saw or other means before it is installed.
Pardon my rough drawing, I hope it gets the idea across. Once set up it will be safe to use. Finger boards can be added for extra safety. The blade is drawn projecting much farther than needed to make the cuts you need. It is the way the drawing came together.
The fence needs to be taller, and the blade only needs to project 1/4" through the fence. The "window" the blade pokes through needs to be made high enough to raise the blade to the highest cut. The table has a raised table added to it so the arbor does not hit the main table, the fence is screwed to the raised table. Use 3/4" plywood for the fence, it is not going to split at a bad time while material is being pushed through. Wax the fence and upper table, it will make the stock much easier to push through.
Make test cuts on same size scraps to gauge the cuts by.
Order of work, before the extra table is assembled, set the saw to cut the shoulders of the tongue portions. All finished cuts will be centered, so if you are using 3/4" thick stock, set blade 1/4" in and a scant 1/4" into the top of the stock. Set up your finger boards, run one side after your test cut is made, flip the piece, run the opposite side. do this with all stock with a few extras.
Next set up the fence I drew earlier set the blade a heavy 1/4" through the fence. Start the cuts for the groove on the other edge of the previous pieces as near to the center as you can eyeball, exact is not important, but get it close. Flip the stock and run it again. You will have a groove started that is perfectly centered. Raise the blade slightly and repeat until you get a 1/4" wide groove. until all stock has been run.
As a reminder, all initial first cuts are done with the test stock first.
Final passes. Raise the blade to get the remainder of the tongue cut, this is a little more critical to cut right the first time. I would actually set the saw at a slight angle, say 1 degree to give the tongue a slight taper, so it will ease the assembly of the pieces.
With the test piece. set the blade so it cuts as close to the 1/4" needed to fit into the grooves cut on the other edge. Run all your stock and make the last adjustment to get it to fit right.
All done!
Note: your results will be better if all stock if flat, straight and uniform in thickness, if not the grooves will change as the wood does, you will be mad at me. Finger boards will help flatten out the stock a little, it will not do it all though.
Good luck
Best Answer
What you described with the 45 degree bevel is called a french cleat. It's used for securing heavy items to walls and I've seen them used in upper kitchen cabinet installations. 1x6 hardwood cut in half on a 45 I think would suffice.
I would consider putting one on the top and one on the bottom (mounted upside down) if you have enough room to slide the piece from the side. That way it eliminates rocking and someone trying to lift it up.
They make metal french cleats for hanging large pictures and mirrors but that max I've seen was a 200lb rating. Maybe you can find some that are rated for your project. Unfortunately they are 20" wide which would be too long to have 2 of them side by side but they do make smaller ones. Maybe there's the right pair for your work.
Here's another idea I had..... Disclaimer... verify that this will be strong enough for your needs. this is a DIY solution with no testing for fitness.
if you can drill into steel. Get some structural steel channels and drill some holes in the backside every 16" to drive lag screws into studs. In the front drill slightly larger holes to allow you to get the lag screws in from the front.
On the top and bottom drill holes for long bolts. Drill the holes just the right size for the bolt to slip through and fit snug so there's no rocking. You'll need to secure them on the inside of the wall mounted channel with a nut. Getting the nuts in on the edges shouldn't be hard. The ones in the middle you'll need to cut some access holes in the front to get the nuts in.
Attach a similar channel with the same type of cutouts (except the access holes for the blot nuts) and attach it to the back of your artwork.
Make another pair for the bottom or at the very least attach a piece of channel as a spacer.
Attach them near the top and bottom so that you can reach in with a nut driver extension to secure the bolts.
If you can't or don't want to work with steel here's an improvement to the french cleat that will provide some more support to help keep the artwork from lifting up and moving side to side if bumped and doesn't require reversing the bottom cleat.
Make the cleats as you normally would out of 1x6 lumber but make sure it's a couple of inches shorter than the width of the artwork. To the upper cleat attach a 1x1 or 1x2 strip of wood that is 5-1/2" long so that it's flush to the top of the cleat on the artwork. Do this for both the top and bottom cleats.
After you slide the artwork on the wall mounted cleats, predrill and drive some long wood screws into the sides of the bottom cleat. I only did 2 screws per side piece but should be room for 4 on each side.
This way the artwork won't fall, lift or slide to the sides.