Bowler Performance is where I would look. They offer tko's, t-56's, and also sell the Legend LGT-700 5-speed which is what I would be looking at if cutting the tunnel is your main concern. With all that being said I have one of Mark's modified close ratio magnums that has been flawless for 3 years of abuse, in fact it's the only part of the driveline that has not been blow-up or broken.
http://bowlertransmissions.com/