the base ring is thicker and slightly wider on teh TPI dist. gm consider this a "heavy duty" piece which is why. the parts crossover issue is mostly for earlier TBI intakes which were cast and cut with a smaller base for the dizzy to sit in. i believe they changed teh casting somewhere around 1989 or 1990, the later intakes are also supposed to flow slightly more and have less casting flash thanks to updated techniques.
