Well, having driven both the 340 and 383 Formula S fast backs, there was not a lot of difference , the 340 handled a little better.
Both cars had 3.23 suregrips.
What really woke up the 383 I drove was when fender well headers were added. (all that was available back then)
Now that was a marked improvement as the stock manifolds, especially the drivers side was very restrictive.
The 340 seemed to rev faster but maybe that was just the difference in sound of both motors.