Scott hello and welcome to the forum .
Would this be your first gun ? I only ask because some of the questions you ask lead me to think you may not have too much experiance ? If I am wrong ,I appologize !
Any mortar the size of what you are talking about is insane .
(this coming from a guy who wants to sail around the world)
If you have the coin just buy a herns 8" (bowling ball mortar) and be done with it ! stick it on a trailer like Tracy did with his big mortar .
Of course if you got the coin ,,, the big one may be avaliable turnkey package deal .
bowling balls are cheaper by the doz .
I have a mortar that shoots pool balls I know the owner of a poolbar and get wore out ones free . casting custom 10" balls is .......
the differance between the wow factor will be nonexistant ! Herns make a darn nice 8"er
http://www.hernironworks.com/siegemortar.html YMMV Gary