If you don't mind me asking. Why would you swap a (as you say yourself) working fine gearbox for no real reason? unless your running huge power and doing loads of track days/drag runs a std gtr box should hold up fine as long as its well looked after/oil changed regularly. If you are planning on big power maybe get your own gearbox rebuilt with stronger internal's
click \/\/\/
http://www.ppgearbox.com.au/page.asp?categoryid=12

not having a dig at ya or anything just genuinely intrested