Figuring Retrofit

CRAFTS need additional fish tank, in my current setup I only have one. I will add one additional tank then will add another, but before able to do that I need to modify this current rack making it possible to accommodate the fish tank.

Currently its 36"W x 24"D x 27"H, I need to modify it to 39" x 21" x 27". It need to be wider by 3" making it 39" correspond to next cut on angle iron and reduce to 21" from 24" to ensure I did not need extra piece of angle iron to make this rack. If "D" remain at 24" I will not be able to cut a 10 ft angle iron to make the pieces. I will probably waste another piece.

Before I can do anything drastic I will need to wait for this bed of Kailan harvested.