These two trucks struck a cord for me, SramblingMan! In the summer of 1966, I worked at Mono Village/Twin Lakes out of Bridgeport, California. Norman and Alpha Annett owned the property, and Norman was a hard worker. He liked to work alongside his help, and I was a wiry 17-year-old from Gardnerville, Nevada, getting in shape for high school football by operating the resort's flatbed garbage truck with David Warr, a high school graduate from Yerington.
We enjoyed unloading the Annett's new white Loadstar 1600 van box truck when Norman came back from Reno with pallets full of cement blocks. The game was to grab two blocks in each hand and empty each pallet in minutes. It was amazing how much weight that Loadstar carried!
For me, the Loadstar, with its bulldog front styling that you describe, was the epitome of a "tough truck". With its five-speed transmission and two-speed rear axle, it was a blast to drive. I believe the engine was a 304 I-H V-8 in crimson I-H red, and I was sold. Yes, I know your two trucks!
As for the carburetor and other issues, the '69 is from the conventional breaker point era, the '78 should have a more modern electronic ignition. Both use a carburetor with a governor, and I-H was quite fond of Holley. I have every bit of tuning data you will ever need in my library, plus every kind of engine, transmission, brake and axle detail. Feel free to ask, happy to provide details!
Engine stalling and any other quirks can be addressed. After five years, all fluids will likely need attention. The fire truck will require basic tune work, you'll have to see what the '78 yields. Before doing anything, I would explore the engine condition of each truck. I've talked in these forums about a cylinder leak down test and other diagnostic approaches. We can take it from there. Make sure each engine meets these four demands: 1) normal compression seal, 2) correct valve timing, 3) correct valve lift, and 4) normal oil pressure with no internal noises. At that point, all reliability will be tuning-related and eliminating any vacuum leaks.
While outside the "light truck" realm, we'll make an exception here. For I-H owners, the Loadstar powertrain is not far from options available in the bigger pickups, Scout II and the Travelall. Few know this, but the Travelall optioned a 5-speed transmission that was similar to the Loadstar. That's what I like about International-Harvester trucks!
Just ask, I'll be happy to assist...