Originally posted by GNVAIR
All the 78-87 A/G body cars are what are considered a metric chassis. But GM had a very strange way of phasing in metric fasteners.
78 and 79 models still have 7/16 wheel studs and standard threads on their power steering pumps and steering boxes.
80-88 had 12 mm wheel studs and metric threads throughout the rest of the car. GM also changed the spindles and rotors on the 80-88 cars. No idea why, but the shaft of the spindle is longer. The only areas where you wont find metric is on the engines and earlier transmissions. The later model Buick V6 and all the 200c and 2004r transmissions had metric threads on many accessory holes although many pipe fittings such as trans cooler lines were still standard threads.
I have found that much of the body and frame hardware on the 78 model year vehicles is larger than the 79-88 models.
Back to the original post.
If I were to do such a frame swap I would do this
78-80 cars would get another 78-80 frame
81-88 cars would get another 81-88 frame.
Exceptions to the rule would be the 81-83 Malibu and 81-86 Lemans/Bonneville which could get away with a 78-80 frame since they were still the same. The differences were with the Monte Carlo, Cutlass, Regal and Grand Prix which received longer front rails starting in 81.
Oh yeah! El Caminos uses a different frame than all the other 78-88 A/G bodies since they had a wheel base that was approx 10 inches longer.