The 64 Riv had a ST 400, but it was not a switch pitch like the 65-66 Nailhead ST 400. They are dimensionally the same size and bolt in the same location using the same mounts. The fixed pitch converter from the 64 ST 400 will not work in a later ST 400. The wiring will also be different. You will need the stator idle switch (for stator positioning at idle) and detent/stator switch (for downshifting and stator positioning under load). These can be found on any 65-67 Buick with a ST 400 or ST 300. You would just need to adapt then to your car using the same brackets or modifying them.