Your just not getting it. The is no reason to remove that return line when you put a holley on the car, or even if you removed the stock fuel filter whcih is where the connection to the vent line is located. Lets take a poll of all the people on this forum who have put a holley on a car equipped with 2 fuel lines, (L48, LM1, L34 and L35s) and see who pulled that second fuel line out just for a carb and or engine change. I had a holley on my car 15 years ago and I still have both lines. Good thing cause I have a stock rochester on it now.
The real problem is that the engine is coded as an L35 and the date is correct for the cars build date. If the car was an L78 originally and some previous owner put an L35 in it because the original engine blew up, is it very likely that the he (or she) would have gone thru the effort to find a correctly dated Camaro L35 block, or would he have just put any old big block he could find and build it the way he wanted it? If someone puts a holley on their car they just make a new fuel line, they don't have to buy one from GM, but they could buy one from an L78 if they wanted to. There is still no reason to remove the return line from under the car, most people would just leave it alone, and at most put a cap on the end of it.
Its the date coded L35 that is the root of this problem. If we can accept the fact that GM would never let a single fuel lined L35 out of the factory, and assume that 99.9% of all camaro owners would not replace the fuel lines when they put a holley carb on an engine we end up with the conclusion that that engine does not belong in this car. We then take the next step and then assume someone attempted to clone a matching numbers big block from something else. There are very few 69 Camaros that had a 3.07 axle as one of its three standard ratios (economy, standard and performance) and those cars are the LM1, L48, L34, L35 and L78. Four of those have dual fuel lines, one has a single fuel line. Thats not to say someone couldn't order it as an optional ratio on any car but the 3.07 is not exactly something you would pick if you were selecting an optional axle ratio. So if we further assume that the axle is original, the single fuel line is original, the car had power front brakes then the car begins to look like an L78. BUT is has a date coded L35 in it and theres the problem.
I'm just playing the odds that while engine swaps are extremely common in these cars over the past 36 years, people swapping fuel lines is extremely rare unless the car is being totally restored which it sounds like this one hasn't been. Thats why I rely more on the fuel line being original than the engine code being original.
So do we have someone (previous to the current owner) attempting to make a matching numbers big block out of a big block and just not being smart enough to know they were replacing an L78 with an L35, or do we have someone just a little smarter and they replaced (or restamped) the engine transmission and rear axle with correctly dated parts and putting it in this car. I don't know, everything is speculation atthis point with the info we have at hand.