I have two MKllls and each is in a double arm plinth. I believe you have room for two arms as well on that Porter Plinth. I use some high end arms and cartridges. The PC-1 Supreme on the 312s is very good. I currently also enjoy the Benz LP-s on Reed 3P. Albert sells a really good imported unipivot you may ask him about. I have a Shelter 90x on that one and even the Ortofon M20 fl super is great on it at $175. No experience with the Atlas unfortunately.
One of the spots I reserved for the EPA 500 system which is somewhat dated and not as fully functional as the others but sounds surprisingly good. It's also affordable and very flexible. I rotate armwands in about 30 seconds. It's a great way to work in less expensive MM cartridges(Ortofon M20 fl super, Empire 4000 D/lll, Shure V15vxmr) or a mono cartridge(XV-1s mono). They all sound great.
Anyway, you should be able to set this up with performance and flexibility that will push the limits of most budgets. Have fun.