how much difference is 1/2" behind the port?
 

I've made my port 2.5" x 16.5" now I'm only a 1/2" short on the back end (I'll have 2" behind the port instead of 2 1/2". This is the same DD 3512 2.7ish cuft after everything in.

if I understand that correct your port is 2.5" wide but you choked it down
1/2" because you didn't bend the port at the 2.5" mark from the back wall

you have to keep a constant 2.5" thru out the whole port or you will alter
the tuning and maybee even add port noise in your case

No i have a 15 1/2" port now instead of a 16" with 2.5" under it its 2.5" wide. Specs say to have a 16" depth port, Whats my tuning going to be at? Drastically changed?? Worst case I can build a 1/2" baffle.

sorry I mis understood your wording

anyways your tuning would be slightly higher with 1/2" less port length
the double baffle would fix it

like he said, the tuning slightly higher, but it wont be anything substantial that your ear will be able to notice.