When it comes to rust the A-body was very solid. Instead you should spend your time checking for collision damage. Check the rear bumper mounts, trunk floor pan and rocker rails for wrinkles, signs of bondo or non factory welds.
Verification & Documentation: Since these cars are popular with hot rodders, you must check for clones and back yard V8 conversions. Real 340 Swingers and 340 Dusters came with a complete drive train and package to handle the extra torque a V8 places on the body. The 340 models have extra reinforcements welded around the leaf spring mounts facing the rear floor pan. Regular 318 and slant six cars don't have these reinforcements. It's common now to find these supports welded on to duplicate a factory 340 car. The factory spot welds are usually neater in appearance compared to a clone. Be forewarned I have seen very good examples. Some cars have entire leaf spring perches installed from donor cars. I have even come across a 340 Swinger that had an entire 340 rear clip welded n place.
The V8 engine K-frame is also different compared to the slant six K-frame. The 340 engine is paired with a 8-3/4 inch rear axle with special 6 leaf springs. The torsion bars are 0.87" in diameter. It should be noted that after 1970 the notchback body style was not available with the 340 engine and these items won't be present.
The VIN number tells you what engine was installed from the factory. Check the fifth digit for an "H" which was the 340 four-barrel. The 318 two barrel V8 used the "G" code. Slant six engines used the "C" code. This VIN number is found on a metal tag riveted to the driver side of the metal cowl part of dashboard. You can see it through the windshield.-
All the cars came with stamped metal fender tags which confirmed the drive train, body and interior colors, performance and luxury options. This fender tag was installed on the driver side fender apron near the shock absorber tower. Check it to see if the VIN matches the one found on the dashboard and driver side door. You can consult a suitable decoding book to confirm the other options your car may have. While you're checking out the VIN numbers, inspect the rad cradle area for a partial VIN stamping. Reproduction rad cradles have no VIN numbers at all.
The factory stamped the last six digits and some of the preceding digits into the top rail where the radiator bolts in. It may be reversed so a mirror may be necessary. Sometimes on unrestored cars, it will be hidden by the antifreeze decal. This confirms two things; the car wasn't in a major front end collision and if it matches the other VIN numbers you have the original driver's door, cowl and front clip. This is the minimum it confirms because with 340 Swingers and Dusters, these cars are desirable enough to cobble together fakes using parts from donor cars.
The engine and transmissions are stamped with VINs as well. Check the V8s on the machined area just above the oil pan for a matching number. The slant six VIN is stamped on a machined pad found beside the oil filter and underneath the last two spark plug holes. On automatic transmission cars, the dipstick tube brackets the location nicely so check between the tranny dipstick and oil filter on those models.
Transmissions also have the VIN number stamped. On the automatics, check on the passenger side of the bell housing near the engine oil filter. The manual transmissions have a raised pad very close to the input shaft area on the passenger side of case. If the VIN numbers match, you have the factory original drive train. Following these hints will net a sweet Mopar A body.