72 R-code equipped cars will have all of the following....
"R" in the fifth place in the VIN, denoting the 351HO engine option.
"V" under *axle* on the door data tag - denoting the 3.91 Trac Loc rear axle
"5" under *trans* on the door data tag, denoting the wide ratio 4 speed Toploader transmission
No other transmission or rear axle codes were available, the 351HO was also not available with factory installed air conditioning.
R-code engines were available in any body style or model in 1972, unlike 1971 with the Boss specific model. They carried some unique parts...
D2ZF-GA Motorcraft 4300D carb
D2ZF-JA Motorcraft dual point distributor
D1ZX-CA aluminum intake
B302/B351 style finned aluminum valve covers w/drippers
4* marked heads with screw in studs and guideplates for the 351HO mechanical camshaft
D2ZX marked damper, approx 2" thickness
"N" case rear diff w/WFD-D or D2 axle tag code and 3L91 stamped into the ratio location, staggered shocks and rear sway bar.
Boss 351 used D1ZE-BA & D0AE-G 351C-4V exhaust manifolds, which were machined to 2 1/4" openings, the 351HO most likely used them as well.
To be "numbers matching" and to guarantee that you have the original engine and transmission, you need to find the partial VINs stamped on the rear of the block below the driver's side cylinder head, and on top of the d/s ear of the transmission. The stamping should read 2Fxxxxxx, they do not include body or engine codes.
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