It's entirely possible '69 cars didn't get heat shielding. I was basing my comment on later-year dual exhaust installations.
The wild temperature swings are probably the catalyst for your tank vents' behaviour.
Swapping to the '71 tank is probably going to help your problem quite a bit. '71 was the first year all cars in North America (not just California cars) were equipped with evaporative emission control systems (ECS). The inner tank was used on '70 California and '71 cars only. It is at the top of the main tank and connected to the main tank chamber by a very small passageway, so that the main tank chamber fills up with the secondary chamber still mostly empty. The gas pump nozzle detects a full tank and clicks off, you hang up the nozzle and drive off. Over the next little while, the secondary tank fills up from the main chamber, dropping the fuel level in the main chamber so as to prevent fuel leaving the tank when things warm up and expand. (For '72, the inner tank was deleted; by clever shaping of the top dome area of the tank, the same function was achieved by means of a controlled air pocket at the top of the inside of the tank).
Note most '70-'71 ECS fuel tanks have four vent fittings, one at each corner. These were brought together in the trunk to a more-or-less vertical vent cylinder that usually leaned against the left or right wheelhouse. You may have to creatively rework your vent system in your '69 to interface with the '71 tank. While you're at it, inspect the present vent system to make sure none of the lines has been crimped or kinked, and keep a sharp eye out for rightside-up U bends in the line, where liquid fuel can condense and pool. All U bends should be upside down. It may pay dividends to replace the longer lengths of vent pipework with flexible fuel hose, which has less tendency to condense fuel vapors that hit it.
Also be advised that the '71 tank would have been designed to work with a Pressure-Vacuum fuel cap, rather than the solid cap your '69 has now. I don't know how you intend adapting the '71 tank to the '69 car, so I can't advise whether you should try to get a '71 filler neck and cap.
The '70-'71 ECS was also unique and interesting in that it used the engine crankcase to store fuel vapors piped from the fuel tank and carburetor bowl. The interface point was a fitting on the fuel pump body on slant-6 cars, and extra fittings on the engine breather cap on V8s. When the engine was started, the collected vapors were drawn off via the PCV valve. This system did a reasonably effective job of containing the vapors, since the crankcase was well sealed, but caused hideous hot start/hot idle problems, since there was no control over when the vapors were and were not extracted from the crankcase. Most of the unburned hydrocarbons wound up out in the atmosphere anyhow, having been first put through the engine (and emitted as black smoke).
For '72, things got very much better. The crankcase was no longer used to contain fuel vapors. Instead, a cannister containing activated charcoal was mounted at the right front corner of the engine bay. Lines led to it from the carb bowl and the fuel tank vent. There was a "purge valve" mounted atop the cannister, with a small diameter vacuum line teed into the distributor vacuum advance line, and a large diameter line from the underside of the purge valve, teed into the PCV valve hose.
There's vacuum in the distributor vacuum line only above idle, so the purge valve only opened above idle. Voila, no more hot start/hot idle problems, since fuel vapors were only drawn off and burned above idle. This system works very well and is still used, almost unchanged, to this very day.
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I've already got a vented cap on it (although non-vented was how it came equipped, as you are aware).