This is very similar to my setup, which I use for my 55gal tank and my 2x 30gal, 29gal and 20gal tanks.
The difference is I used 2 x 4's instead of plywood.
First, there's two 2 x 4's on the floor. Then a stack of 3 cinder blocks, then 4 (I think) 2 x 4's side by side on the top where the tanks sit.
XXXX <- More 2 x 4's
OOOO <- Cinder blocks
X......X <- Base 2 x 4's (periods used as place holders)
This setup has worked fine for 12 years and counting (please, no one yell jinx!! )
In retrospect, I would used something more then 1 layer of cardboard on top of the 2 x 4's and under the fish tanks. But, that's all I could afford at the time.
As for you setup..
What thickness of plywood would you use? 1/2 inch? 3/4 inch?
How do you plan on arranging the cinder blocks? ||| or |-|
1/2 plywood would probably work okay with |||
3/4 plywood should work okay with |-|
Then again, this could all be overkill or a disaster waiting to happen, see what other people have to say.
Also, my response is my opinion. I'm not structural engineer.