A blessing or a curse?

I built my car in 1997>99. At that time, metal had just become available for my 68 Barracuda. So I spent my life savings and got the bodywork done.
Six years later, the new rear quarter started rusting in all the usual areas, WHILE, the 38 year old quarter on the other side, was still looking good.
Sheit

These are just some of the used parts I used building my Dart.
5.9 short block
Intake
Fabbed al valve covers
Holley 770
Headers
Clutch fan
42RH trans
Converter
Flex plate
Rear sure grip 3.55
Rear disc brakes
Alternator (from the 318)
Water pump (same)
Timing cover (same)
V regulator (same)
W washer motor (same
Recovery tank (same)
HEI (same)
Recurved Distributor (same)

That’s some of it.