Plan for your snake to get bigger than the largest boas recorded, then you never have to be between a rock and a hard space should it out grow the normal sizes.
Also are you 100% sure yours is a male? I have had my boa for 6 years next month and have never had it sexed. Because of her size I am assuming she is a female but there are some large males out there.
I would make the cage longer, 6'L x 3'w x 5T sounds great. Keep in mind many people house full grown boas in cages smaller and/or shallower but If you want to go big that is what I recommend. Our boa lives in a slightly taller cage than that.. It is constructed of medium density fiber board (mdf) and wrapped in 30ml pond liner. Here are a few pictures of the cage, the round black bowl is a barrel liner for a water dish.
For the rock wall. You can find many ways of doing it all over the web, My advice would be a mortar or a fiberglass and sealed with a polygem dino goo epoxy.
