J78,
Good question-
It looks like a matter of things stacking up:
* I have your weight down as 3310, almost 300lb heavier than the 68 Camaro.
* Your T350 helps, but you also have a taller rear - and the T350 does draw a little more power than the Glide.
* The engine is down maybe 15-20ftlb across the board, a matter of smog tuning mostly.
That pretty much does it.