Like I said, I change my plans and I want to know if a 2.5cuft ported box tuned to 35hz is a good box for a single 12' dcon. Too big, or ok box? Tuned too high? I'm going to send it 400wrms.

It is just a hair out of our suggested specs. If the Dcon feels like it is unloading too easy, you can glue small wood blocks inside, but I am pretty sure you will be fine.

Should be ok. Does your amp have an adjustable Sub Sonic Filter (SSF)?

Yes, my amp have a sub sonic filter. on off set at 35hz

Ok, small wood blocks...but: how many of them?? Thats the problem