G86 is a closer variant to the common audi boxes you guys use. G96 six Speed is a MERWD rendition of the 996 gearboxes. Bigger gear widths, LSD options, stronger casing. However both can be fitted. Boxster guys have been swapping in the S box in place of the five speed on the older cars with nothing more than clutch/axle/linkage swaps. G96 bell pattern is different(few additional fastening points, but still bolts using common bolt locations over 85% of the pattern. Similar to bolting Sti Suby six speed into the older five speed cars(IE 2.5RS).......six speed has 3 more bolt locations that are used on the newer engine cases, but still share 6 common bolt locations and the two drifts/dowels for alignment and securing..
Of the two, get the S box. In the Porsche community the five speed is proven for a good 300-350ftlbs before things start flexing. S box/cayman box is good for 450-500ftlbs in a ~2200lb vehicle with ease. Many modded Caymans and Boxster S's coming close to pushing that power for extended durations, and they've got some fat on top of our weights here. Over that and you need to look into metal treatments to the gearset and main/pinion shafts.
Worked with Ryan @ GBOX a few months back to get a dummy G86 case for mockup here(got the cases), but soon ditched that plan of attack in favor of a locked out AWD Suby 6 speed to take the punishment once I found G86 failures above 300ftlbs were quite common(gear tooth breakage). Suby 6 good for 500wheel with a front LSD upgrade, 7-800 if upgradced to PPG gearset(dog)
-Adam Hennessy
R247 Motorsports