Bob,
Here's some info from a previous related discussion here:
http://www.348-409.com/forum/showthread.php?t=16053&highlight=build+date
"If your trim tag doesn't have the body build date, then you have to look for other clues beginnning with the casting dates on the engine block, heads, and any other dated components in the engine compartment as well as the rear axle center section to see if all the dates are reasonable. There are other dates to look for like on the back of the speedometer and/or clock where an assembly date ink stamp may still be visible. Any original glass shoud have a 2 letter LOF code which can also be a clue to when the car was built. Unfortunately I haven't seen any list of end of month last built VIN numbers for these like there is for the Corvettes and 55-57 Chevy so one could only estimate the build date by estimating the number of units built per day, week, or month. The regular passenger car rate was well in excess of 500 cars a day in some plants, but the El Camino was somewhat of a limited production model and the daily production rate should be much lower and more sporadic so it would be hard to estimate. I believe there were somewhere around 15,000 built in 59.
Other clues should be a 3/8ths fuel line which should go under the right front A-arm although a 4 barrel 283 would also have that as well. 348 would have dual exhaust and the fuel line should run on the outside of the frame then cross over to run inside the driveshaft tunnel...again a 4 barrel 283 would be the same but a 2 barrel would have single exhaust and would not run in the tunnel.
The wiring harness could also give clues.The 59 had the resistor built into the harness so no resistor on the firewall unless it was a solid lifter engine. The 348 cars came with a 61 amp/hour battery and the others had a 53 amp/hour batt. I suspect that because of the extra amperage the hot wire from the positive term to the voltage regulator was probably 10 guage instead of the standard 12 guage. The 348 voltage regulator mounted to the radiator support using ONLY the top two holes with a rubber shock mount replacing the bottom mounting screw....all others used three mounting screws.
The 348 cars came with 3:36 rear on standard trans and 3:08 on automatics.
I'm sure there are other clues and others will probably chime in with additions/corrections."
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