Engine power problem

[mrr1999]

My 70 Chevelle has been in storage for the last 3 years or so and I am trying to work out the bugs in it. I drove it over the weekend and found the car runs fairly smooth to about 50 MPH (2200 RPM) and suddenly hits a brick wall as far as power goes. Car starts shaking badly and will not go any faster nor will it rev any higher.

Any idea what I should look at first? It is a fairly stock 396 with a TH400.

Thanks.