Thanks for the encouragement, guys. It helps as I weed my way through this labyrinth. This being a "flash" boiler means a number of controls that conventional boilers don't require. But I'm figuring it out.
Here's the new and improved boiler coil. I upped the size from 1/4" OD tubing to 3/8" OD. I did this because it works better with my 1/4" fittings and the engine is bigger than anticipated. This reduced the length of tubing from 50' to 35'. It's a lot more difficult to make a coil that fits inside a 6" pipe when the tubing is 3/8". Still, I tried to create something of a beehive effect for the lower half and a concentric circle effect for the top half. I've decided to put a 3" piece of pellet stove pipe up the center of the coil to force the heat from the propane burner to flow over the coils and not exhaust up the middle. At mid-point in the coil is a high pressure, high temperature compression T-fitting (not easy to find). A sensor will be fitted into the T on the outside of the boiler to detect water level in the coil. This will run through a little circuitry because there needs to be a delay in the switch so the water pump isn't surging every time the water in the T bubbles up and down.