I remember having to line up the blocks first and swap them around as they were slightly uneven in size and it took a bit of shuffling to make sure they were even. I no longer have the tank and blocks so can't look but I am pretty sure it was chip board, not ply. The base was 1.5 - 2cm thick. The blocks are 19cm wide so the gap I had between them would have been about 22 - 25cm.
That tank ran for about 4 years sitting on those blocks. I had a 3ft in the lounge, also on blocks, that was set up even longer.