I was going to convert our 1987 Turbo so it would be more competative at autocrosses, etc, but went out and bought a 914 instead. I was going to have Patrick Motorsports in Phoenix AZ do it and the cost is roughly $7500 with a limited slip and they do all the work. Takes them three to four days. Their web site is:
http://www.patrickmotorsports.com/
One down side is higher RPM for the same speed unless you go with custom gear sets, etc.