RB engines were mated to different gearboxes
* FS5W71C - same as the S13/S14 Silvia gearbox, except with RB bolt pattern
* FS5R30A - RB26, RB25DET and Skyline 4WD applications, stronger and heavier

71C gearbox for RB20 can be used with other engines (e.g. CA18DE, KA24DE, SR20DE, VG30) by partially disassembling the gearbox and swapping the front half of the case it can be used on the other engine.

If you can find the 1200 tunnel from a 71-series automatic (L3N71B), it will go into the ute and you can still use the heater. You don't need to change the tunnel, but it makes gearbox installation easier.