Sounds like it might have been a bit of a sloppy install. None/few of the army fitted holdens seemed to be done "nicely" See if you can tidy it up somehow.
Mines a 73 SWB s3 with a 186. I added a toro overdrive, and it sounds better at over 50 km/h however the top speed hasn't really changed. I got 91 km/h without OD, and 94 km/h with.
Mind you - my engine is tired too.

