There is a factory alternative to the E30 M3 Guibo in the form of the similar
unit from the E34 M5.
As is clear from the photograph, the E34 M5 piece is substantially thicker. This is
especially so in the "web" region between the six mounting bolts. This is the region that
carries most of the torque from the transmission output shaft to the driveshaft.
On the E34 M5 Guibo the web region is 21.0 mm thick vs. only 15.5 mm for the stock E30
M3 mount. Also, the region of the bolt holes is thicker on the E34 M5 Guibo
(35.0 mm vs. 30.0 mm). The 5 mm extra length is easily accommodated by the splined
section that connects the two driveshaft halves.
The list price for the E34 M5 Guibo is $95.75 but you can find it cheaper if you scratch
around a bit.
The part number for the E34 M5 Guibo is: