Q: how many zillion of dollars do you need for a s15 swap?
A: waaay too much for most people.
If you ask me, you are going to have a hard time. That's quite a bit of modification involved in getting that 6 speed to mount properly in the S13 chassis (as you have).
You'd probably been better off getting a S13/S14 and then paying for the NISMO 6-speed w/ associated tidbits (which is stronger and closer ratioed), and bolts on to the S13 chassis (speedo works like it should, etc). It'll land you at probably a little less cash if you do the work yourself, versus the S15 clip, which you can't do yourself unless you are good at fabrication.
To be honest I'm really jealous that you have a S15 clip in your backyard!!!

You'll probably be only one of the handful of people in the US that has done the swap. But have fun doing it, you'll be very happy with the car when you are done. Let us know how it goes!!!