a 6' x 18 75 gallon tank would be only 13" tall. I am a proponent that more area is better, but asthetic considerations also apply. A front window that is 72" by 13" tall is a little short for the length. The low hight, being only 3" taller than a 10 gallon tank would change the max fish size imo. It signifies a more intimate viewing distance and thus the fish to match that viewing distance.
One might be able to have a number of pairs of dwarf cichlids or space for breeding small plecos. Several colonies of African rift lake shell dwellers.
A 125 is 72" by 18" and has a hight of 22" which is more suitable for the types of fish one might keep in a 75.