I went totally OT with the engine in my car... but it is far from slow. Automatic cars were geared differently than the sticks, stick cars usually came with the 3.56 rear.
The best running Y-Block isn't a Hot Rod by any measure until it get's a little Help. In healthy stock form they still run out pretty good. Give the motor a physical and see what you have. A compression test and checking the timing chain will generally give up if it needs internal work. Good compression should be 95 to 105 and a healthy cam chain will yield about 3 degrees of slack. More than that and it should be replaced. A well worn cam chain will also give false compression readings. I always check the chain first. Read your plugs, that's another give away. The Wizzard
Stock issue Y-Block should show around 150psi in good shape, they didn't have very high compression ratio. 8-1 or somesuch. A well worn chain would skew cylinder compression test #s to the low side I'd think? On the other hand should able to dial in lots of ignition advance without complaint even with pump gas. That should wake it up a little for you. Get into the distributor with a timing light and map the curve out and see where you're at. This link should be helpful: http://m571.com/yblock/distributortuning.htm It's almost always the case by now for an original crank vibration absorber to have lost its clock by the outer steel inertia weight slipping. If the damper or absorber is original it should be replaced. Check compression, verify "true" TDC with a piston stop. Then setup the distributor mechanical and vacuum advance for your engine.