Enclosure ?
 

I just ordered 2 Sundown Audio SA-12's, they recommend the box be tuned to 35hz, my enclosure is the right volume for these subs but is tuned to 38hz. Will that little bit of difference effect the performance of these subs much? Will it still sound good?

The tuning difference should not affect the performance much, if at all.

Won't really affect performance but will affect how it sounds. I had the same set up last summer but I prefer a lower tuning due to the types of music I listen to. Even with sufficient power, I found the SA line struggled a bit to hit the lows from 26-30hz. Don't get me wrong.... they played the frequencies but not with the same authority and ease as some of the previous drivers i have had. GREAT sounding subs though when dialed in to your liking and lots of output. Enjoy them!!

x2 on the above comments.

I need help deciding what size and brand of sub to buy for spl contests and what kind of boxi should build

Since you didn't give any information about your goals, power and car, do 4 18"
Tantric Sounds HDD in a isobaric triple chamber dual reflex 6th order bandpass, should net you mad dbz in contests.