First Vette, a 75 L48 auto given to me by my Brother. It has been sitting under a carport since around 1992. It currently doesn't run so I have lots of work to do. Body is in pretty good shape. Full tank of gas since it was parked so I am in the process of siphoning that out and preparing to put the car up on stands so I can get a look at the undercarriage. The animals got inside and I am not sure of the condition of the wiring. My plans are to get it running and driving and do a restoration as I go along replacing what it needs as time/$$$ allow. I am planning on returning it to stock (mostly) condition.  I have been researching a little bit and it seems that it has a rear bumper cover from a '76 and also a hood from a 77 or later. I will eventually replace these with the correct parts. I bought the assembly manual as well as a Haynes repair manual. So my first questions is I have been seeing engine wiring harnesses in the catalogs. What is the difference between a harness with seat belt interlock and non interlock? How do I know which one my car came with? Thanks.

Most of the early '75 cars did have the seatbelt interlock system, even tho the mandate for them was removed.
If you look under the hood on your car, and the passenger side, above the A/C-heater box, IF the car had the interlock system, you will see an orange connector bolted to the firewall, with a relay inserted. Also, if you look in the very back of the rear storage compartment, on the driver side, under the carpeting, you will see the electronic "Logic" module if the car had the system.
And finally...if it has the system, and the system is working, you will not be able to start the car without first sitting in the seat, and fastening the seatbelt...in that order.
hth

I will have to go out and look for it today. I don't recall seeing the relay over the ac/heater box so I will have to look in the back under the carpet. The car has been hacked up somewhat with lots of things removed. I want to put all of it back to factory correct. I am not looking for a show car but a good driver with all of the correct parts/options.
Another thing I don't have is the air pump. The corvette restoration guide says that some but not all 73-75 small blocks had the air system. The car has headers on it so the manifolds are gone. Doesn't look like the bracket is there for the pump so how do I know if the car had the air pump? 

The small block automatic cars('74-'75) did not have the air pumps, iirc.
Look to see if your seats have a wiring harness coming from under them....you should be able to see it with the seat pulled forward, and tilted forward. If you do not have the seat switch harness on either seat, your car prolly did not have the interlock. It was mostly only installed on the early '75s. The orange connector with the relay is usually a good sign it had it at one time, but not a definite. The wiring harnesses could still be used, they just dint install the electrical stuff like the relay, or the logic module. They would use them until they ran out of them, and then use the newer, updated harness.
What is the build date of your car? That will help narrow down the possibility.

I will have to find the build date and investigate further. I assume that they all had cat converters so I thought that the converter needed the extra shot of air from the pump to do a complete burn of the spent gasses? should  I buy the exhaust manifolds with the plugs or did they use manifolds with no holes on the small block cars without the pumps?

|UPDATED|2/4/2014 2:16:39 PM (AZT)|/UPDATED|

I found the numbers on the front of the engine as well as the data on the driver's side hinge area. The sequential numbers for both plates match so it looks like this engine was the original engine for this car. The door area plate indicated E03 which if I have it right states that this car (body) was built Feb. 03 of 1975? Would this be subject to the seat belt interlock? There are two small bolt heads above the engine compartment heater core box. Would these bolts have been put there in place of the seat belt interlock module or should this area be smooth and free of any fasteners? I found a relay and flasher in the rear storage passenger box (the one with the jack in it). From what I have read this is part of the alarm/security system. Also there are two wires near the passenger female seat belt buckle but they are not hooked up to anything. I am also trying to find out if this car had the air pump. Some have suggested that the l48 auto cars did not have the air pump but I have seen more than a few for sale online and they clearly have the air system installed.

|UPDATED|2/23/2014 6:33:01 AM (AZT)|/UPDATED|

A Feb '75 built car would NOT have had the interlock. The wiring you see on your inner belt latch is for the seatbelt warning light...all cars had that.
Cars delivered to California DID have the smog pump, regardless of engine size, so you may see some with the pump, but small blocks delivered to other states were not required to have them.

If it's not going to be a show or "judged" car, why would you want to put that crap back on if you don't have to?  It's expensive and just garbages up the engine compartment.  The air pump didn't pump air to the cat, just to the manifolds.