skid

I am trying to help a friend pass the California smog test and need some advice. Although I am familiar with electronic controls, I am confused about GM's part numbering system.

The car is a 1987 El Camino with a 305, carburetor and automatic transmission (Vin: "H" / LG4)

The problem began several months ago when the OEM ECM began displaying a constant check engine light. At that time, the ECM had the following codes:

42: Electronic Spark Timing (EST) or Bypass Fault.
12: (not sure if this was an actual error code, according to GM it "may" indicate no reference pulse from the ignition control system - when other codes are present)

Following the test procedures in the GM service manual, he was not able to find any faults. After several attempts to clear the codes by disconnecting power, he went to a local wreaking yard and picked up another ECM and two PROM's removed from "similar" cars (the ECM was from a 1986 Caprice, I believe).

Part numbers from the original ECM:
1228079 FYK
860671 M6!2546883

Part numbers from the original PROM:
DELCO 7965 FYK
519K 8626
6040 06630
CE1090N

He first removed the original PROM from the ECM and replaced it with the following PROM from the wreaking yard:

DELCO FFN 3435
519K 8526 06630 5029
CE1090N

With this PROM installed, the engine would not start.
He then installed the following PROM from the wreaking yard:

DELCO 5793 CJR
519K 8334
3031 06630
CE1086

With this PROM installed the car would run (rough) and the check engine light came on again.
The error codes this time were:
12 & 51: Prom error, or calibration error.

He then replaced the original ECM with the following one from the wreaking yard:

1227169 FFN
868341 M513706000

With the original PROM (7965 FYK) installed the engine would not start.

With the "5793 CJR" PROM installed, the engine would not start.

With the "FFN 3435" installed, the car has been running and there have been no trouble codes. However, the engine lacks its previous performance when accelerating, or climbing hills - and it is doubtful that it will pass a smog test.

To resolve this problem, I have been trying to find another 1228079 ECM and possibly a new PROM. None of the re-manufactured 1228079 ECM's that I have been able to find mention a 3 letter code and none of the 1228079 ECM's that are listed on ebay mention a 3 letter code, while some have 4 letter codes. For example:

1228079 ACHB (from a 1987 Camaro; V-8, 305 (5.0L), 4BC)
1228079 AHWK (from a 1987 Firebird; V-8, 305 (5.0L), 4BC)
1228079 ACHB (from a 1987 Camaro; V-8, 305 (5.0L), 4BC)

One seller on ebay claims that the following two PROM's are OEM replacements for an El Camino (I have not been able to confirm if this is true):

1228079 ACHA
For 1986 305 4 BBL CARB, VIN "H"
For ECM# 1228079 ACHA

1228079 ACFK
For 1987 305 4 BBL CARB, VIN "H"
For ECM 1228079 ACFK

How do I find the correct replacement parts?
Is the "FYK" prefix of the original ECM important?
Does the letter prefix on the ECM have to match the broadcast code of the PROM?