There will be tons of suggestions to just weld the legs on the frame and I have
no doubts this works and is probably the easier way to get the job done.

However, I want mine to bolt on. I've seen several Landcruiser builders use
the bolt on style over weld on so I know it works but I was just curious
as to what ya'll thought of just simply sandwiching the frame with large
square plates on either side. Would this design hold up?....with the bolts tight
to the bottom and top of the frame of course leaving little wiggle room.

Sorry, I only have paint for demonstration purposes.

I too have been thinking of something along the lines of this. It would save the frame from having any holes being drilled through or welds which might compromise the frame's heat treatment strength or finish.

On the brackets, I might have 90 degree tabs/steps (angle iron?) that would contact the frame for a good fit instead of the bolts themselves.

you would have to do some large thick plate. to keep that plate from twisting and wanting to warp around the frame where the bolts are at.

but i think it could work.

That may work for the passenger side but on the inside edge of the driverside frame is where the fuel lines and gas tank reside. It would be simple to slide a plate under the lines rather than trying to fit the angle iron under the lines.

The tighter I get the bolts to the frame the better off it will be for sure

Should work fine. I did an L bracket that goes up against the frame so I wouldn't have to clamp it.

If you clamp it, how tight it is doesn't make much difference (assuming there's no slop). As the slider gets pushed up it'll push the top of the plate into the frame and then pull on the bottom (which will result in the inside frame plate being pulled inward and the top of it not doing anything unless you drop something on it), and then all the bolts will be in single shear also.

With an L bracket the frame isn't being clamped and as far as a vertical load to the slider is concerned all you need are some bolts on the bottom interfacing the bottom of the frame which are then in shear and the long part of the L is just being compressed into the frame.

Food for thought. I then had some angle iron that allowed the bolts to go into the frame and just tapped into the frame. Did long hand and finite element analysis and never had any issues with just two arms.

I have done this, and I can answer this question. Simply put, as you drew it, NO it won't. The reason is in order to get the bolts tight, you'll be squeezing the frame rails and they'll collapse a little, then you'll retighten and they'll collapse some more.

The complicated answer is YES, if you put spacers in the frame like the factory does for the steering boxes on the earlier generations. Also you need to gusset the top, not very much, but just to protect the butt welds from stressing. If you are interested I could take some pictures for you. I've had them on my 7000lb FJ62 for years and they work great! Can't do it tonight, it's dark out and it's a school night

I never thought the frame would collapse. Well crap! I do have another design in mind but it will be more complicated to make.

I've seen a few pics of some SAS and I think I know what you're talking about with the steering box. A picture's worth a thousand words and I'd appreciate it but no problem if it's a hassle.

maybe this?

@slapdappy
Is your truck right hand drive? That picture looks inverted from what I'm looking at on mine. like the idea though.

This is the side view of the design I had in mind so you can see the bends. It would be bent around the frame with upward force being the main factor in design. The only problem being getting the bends just right so they would be tight enough on the frame to prevent any sliding up or down the frame rail.

Not my truck. source of photo: http://www.4x4wire.com/toyota/tech/sliders/slee2.jpg. I like the idea. Get as solid an attachment as with welding while being able to easily remove them. I've been cautioned about sheer force as the achilles heel of bolt on sliders, at least those which bolt through the frame. Doesn't look like your idea would rely upon bolting through the frame so this may be moot. Any likelihood of the brackets allowing fore/aft movement if the sliders encounter enough force? Possible you get hung up on a rock and the slider is forced back or forward along the frame rail? I know nothing of such matters. Just wondering aloud.
I may try the leg-within-leg approach bc I like the idea of removability. I think these would be easier to attach and detach than the frame bracket system most bolt-ons use. Not sure when I'd want to remove them as one of their functions will be to protect against inconsiderate parking lot door bashers, but I still like the idea. I assume the weight gain from six additional legs would be small enough although I am trying to keep weight gain to a minimum.
This process of choosing sliders - kick out vs. straight, square vs. round tube, protrude from body vs. tucked in, delivered welded vs kit form, paint myself vs paying to be painted - is a challenge.