47-55.1 Coolant leak?

[Livemeyer]

I haven't driven my '51 much since I installed a 327 of unknown origin just after I got the truck in 2016. I bought an aftermarket aluminum radiator that fits the 47-54 models and an aluminum capsule-style overflow container. It has two openings at the bottom, one feeds a long tube that runs to near the top and the other is open to the bottom of the reservoir. In theory, overflow from the radiator cap should run down the overflow hose, into the reservoir, fill, and if it fills to the top, it spills out from the long overflow tube.

Now then, every time I take my truck for a drive, at temperature, even a short 1/2 mile drive, when I get home I notice a puddle of coolant below the radiator and on the ground. There are no detectable leaks from any hoses or from the radiator itself. I thought possibly the radiator cap wasn't sealing well, or was too weak, so I purchased a 16-PSI Mr. Gasket radiator cap that also has a thermostat built in. https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
Gauge in the truck cab indicates 195F; this cap gauge indicates 200F.

Still, when I took the truck for a quick spin, I came back and coolant had leaked. Thinking it was the overflow reservoir, I removed it. I capped the bottom, poured water into it, waited to see if it leaked. It didn't. I had a bucket under it, it was clamped into the vice and the bucket underneath stayed dry.

I took the truck for another quick drive, no overflow reservoir, and there was a big coolant mess, it had indeed been leaking out from the cap. "Well I'll show you!" I thought, and I put a plug on the overflow nipple at the top of the radiator. Well it's evening now, the truck has had time to cool down. Both the top and bottom hoses had vacuum compressed, so I realized capping the nipple wasn't the answer.

At this point, best I can do is just put the overflow reservoir back on, but I'm still puzzled about the leak. I don't want to keep having to clean up coolant. What am I missing here?