1958 was the 1st year to have letters identifying the model.
This cut and paste is from the 1958 Cadillac club web site below:
FIRST AND SECOND DIGITS: Identify the model year. (1958)
THIRD DIGIT: Identifies the body style
SERIES 60 BODY STYLE CODE Fleetwood Sedan M
SERIES 62 BODY STYLE CODE Coupe G
Coupe DeVille J
Edorado Seville H
4dDr. Sedan K
4-Dr. Sedan, Extended Deck N
Sedan DeVille L
Convertible F
Eldorado Biarritz Convertible E
SERIES 70 BODY STYLE CODE Brougham P
SERIES 75 BODY STYLE CODE 4-Door. Sedan R
4-Door. Imperial Sedan S
SERIES 86 BODY STYLE CODE Commercial Chassis Z
LAST SIX DIGITS: Represent the basic production sequence
Assumiing you have 2 engines, one came from a 58 Biarritz and the other from a 58 4Dr. extended deck.
David 58PXXXXXX