Is a 1989 Chevrolet 454 TBI intake interchangeable with the old style oval port heads?

yes- in fact, i think the 454 TBI intake is just a quadrajet intake with an adapter plate on it.

It will have the smaller 190 cc runners to match the peanut heads but yes they are basicaly just a QuadraJet manifold with a heater plate under the TBI unit. Edelbrock sells an aluminum one that they claim adds about 20 horsepower due to increased plenum volume and bigger runners.

Larger Dave