Brainstorming is really important at this stage in any build. You really need to sit down and figure out what your goals are. Look at your available room for your tank, livestock you wish to have, the look you are hoping for aqua scape wise. And come up with dimensions that will achieve all or most of these goals.
To me if you have 6' of space then why not go 6'? Your livestock options increase once you hit that 6' mark as well. Your tank doesn't HAVE to be massive gallon wise at this size either as mentioned above even a 72" L x 22" W x 20" H barely scrapes into the 135 Gallon mark. Which is still 85 gallons smaller than your last upgrade.
LOTS of options out there to consider and evaluate once you step into custom territory. I hope you can find a size that will make you happy on every aspect for a relatively long time. And BTW Planet Aquariums is an excellent manufacturer to go with if you do decide to go custom
@Greybeard has a great example of there work in his build thread.