You only need to support the entire bottom of an acrylic tank. A glass tank only needs to be supported around the edges (unless the bottom is thinner than normal, or unusually wide (not your tank)). The fact that it doesn't have the plastic frame around the bottom doesn't change this fact, so you don't need to use plywood and foam on top, and you don't need to add any 21.5" cross pieces between the horizontal supports.
Foam:
The reason foam is sometimes used under tanks is to avoid the problem of having any little high spots putting too much pressure on one tiny point of the glass bottom. Imagine if you put a flat piece of glass on a flat plywood table, but there was a little 1/8" diameter pebble under the tank. The entire weight of the tank would be resting on top of that pebble instead of spread out over the whole flat top, and the glass would break. Now imagine if you laid down a 1/2" layer of Styrofoam between the glass top and the plywood table, still with the pebble between the glass and the foam. The tank would still push down on the pebble with all it's weight, but instead of the pebble pushing back like it did before, it will simply be pushed into the foam. The foam simply absorbs any imperfections in the flatness of the plywood top, so that there are no particular points pushing up on the glass with enough force to break it, and the weight is spread equally over every square inch of the glass. If the plywood were very flat, you wouldn't need any foam, but why take the chance (foam is cheap). We're not talking about springy couch cushion type foam, but a flat sheet of semi-rigid foam like Styrofoam. If you are only supporting your tank around the edge, then just put a strip of foam around the edge, or simply make sure that your wood frame is very flat all the way around. Make absolutely sure that there is no little piece of gravel or anything hard between the glass and the wood, and that all four pieces of lumber around the edge form a perfectly flat rim, so that none of the four is taller than any other (buy good straight lumber, and/or use planer/joiner to get straight edges and uniform widths)
Assuming that your tank will be made of glass, and that the bottom is of the standard thickness for this dimension of tank (1/2" i think) all of the extra stuff you mention above is unnecessary over-kill, and will just make your stand heavier, more expensive, and harder to build. Notice that the design given at the beginning of this thread (with the colored lumber pieces) doesn't include any cross pieces or plywood on the top. (Instead of adding extra wood to your stand, add capital letters to the first words of your sentences, and question marks at the end of questions
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If I were supporting an acrylic tank, then yes to all of the above: Add two 21" 2x4 cross pieces spaced 18" apart, 3/4" plywood covering the top, and either use foam over the plywood, or just sweep it, and the tank bottom, well to make sure that both surfaces are flat and smooth, with no specks of anything between them. And support the end of each cross piece with either a steel joist support, or a vertical 2x2 going all the way to the floor (I like joist holders for this, just take care to mount them so that the tops of the cross pieces are flush with the tops of the long horizontal pieces.)
