Use the 987 6 speed and be done with it. It's a direct bolt on to the Audi engine, and it will bolt right into the 986 chassis with the matching mounts, and you can use the 987 shifter cables. The 5 speed WILL NOT be strong enough if the HP and TQ numbers you quoted are actual. The only tricky part will be the axle shafts since the 987 outer CV joints don't have the same reluctor ring for the wheel speed sensors as the 986 has. If you're resourceful though, the solution for that problem can be found using OEM parts from different years put together.
I'm doing the same thing in my race car, but using an ABZ engine. I originally planned on running the 5 speed since the ABZ doesn't make a whole lot of power, but changed my mind and switched to the 987 speed for the sake of reliability. I don't consider changing transaxles during my race weekend "fun".
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