Help me out guys. I hate the gsr tranny on my b16. I got a good price on a b16 tranny, but grinds in third and was wondering if I buy this tranny can I swap out the gears?

 

The gears are different, you would have to change third and fourth, and possibly the 3-4 fork.

 

yes you can, you will need both gears though (the counershaft and the mainshaft 3rd gears)
No other parts are necessary reguarless of what others tell you... We had an SI 3rd in our GSR tranny for awhile, but recently took it out since it made the 2-3 shift way to short, and the 3-4 way to long. It got us by until we could get some new parts, but at the time we didn't have anything else, and had to make 2 events.
You should repace the 3-4 synchro if you want the grinds to go away.