Lets see. Its the middle of April and you want 12x20 film for May and June. That could be tough. You may have to shoot what you can get, not your first choice. Most of the time 12xi20 is special order.
I would call J&C and find out what they have in stock. They list on their web site that they have some Ilford in stock in 12x20. They just did a big order of Kodak T-max film, and may have some of that. But, I would CALL them.
As for developers, I use d-76, d-23 split and pyro for alt work. If you have never used pyro, then I would suggest you wait on trying it until you have time to really play with it. You have a lot of new things going on shooting the big camera, developing the big negs, etc. Keep the variables to a minimum.
Which ever film you get, order some 4x5 or 5x7 of the same film and do your testing on the small film. You should be able to have it fully tested before the ULF film arrives.
All of the films you mention can work with alt processes. You might also consider J&C Classic 400. It reminds me of Tri-x. If you are going to do platinum, dick Arentz's book has pretty good development times for several films in the back of the book. His time for J&C Classic was spot on with my testing. His times provide a great place to start.