I just saw that thread after I posted. After glacing through it I see that the design is similar to the one I have. I design this stand for ease of assembly also, nothing but straight cut. So you recommend to use 2x6 instead of 2x4 for the center (front to back) support? Also I noticed that the design you mentioned, for the vertical 2x4 at each corner, it didn't go all the way from top to bottom, any reason why? I would of think it's better to have those beam go from top to bottom for addition support.
As far as the X bracing goes, I don't know how much strength it can handle either, but I thought it's would be good to have, better safe than sorry I guess.
The piece you are referring to is what I have been calling a floater, but originally it was called a screw strip by the person who came up with the design. It effectively connects the upper and lower frame together with screws, but the outer parts of the legs are what bear the full load of the tank. I can't really tell you why it doesn't go all the way to the top of the upper frame or the bottom of the lower frame, but ultimately it wouldn't matter other than ease of assembly. I left a 1/2 gap at the top and the bottom, and that took all guess work out of having the thing sit EXACTLY streight and level.
I tend to overdo things sometimes. I was suggesting that you use 2 x 6s for the outer sections of the legs rather than 2 x 4s. To be honest, this is probably way overkill. 2 x 4s should be more than enough to get the job done. I did suggest using another set of 2 x 6s in the middle between the upper and lower frames though for the center support of the stand. I WOULD do this to a 6 foot stand.
The original designer of the stand stated that tanks up to 48" should be OK with 2 x 4s for the upper frame, and tanks up to 76" should be OK with upper frames constructed of 2 x 6s. Tanks longer than 76" should be ok with upper frames construced of 2 x 8s or 2 x 10s (can't remember). All of these designs were supposed to be able to support the loads without any center brace supports between the upper and lower frames.
I would put the center supports in though just to be on the safe side.
I don't want to talk you out of the redwood other than I don't think it is necessary. This frame design was conceived with pine in mind. I don't know what your pine availability is, but I am sure that it would be much cheaper then redwood.
Personally, I would kind of stick with 2 x 6s only because they tend to be streighter than standard 2 x 4s. I believe that the 2 x 6s are actually what pulled my stand together and made things streight and level (in conjunction with the basic design prinipal that I mentioned).