Gear ratios as well as splines are different from V6 to 4 cyl, diff may well be different ratio.
Just keep the two cars till you have swapped all the bits you need to get running.
The essex V6 fitted in SA is vastly better than the cologne imo.
Obviously keep all the V6 engine bits together, the V6 may also have better/bigger brakes, and thicker anti roll bars.
tabetha