I've heard this song, too. I've had some problems like this a few years back, and sure found some rust in the crudy old shut off valve. I expect your problems will return, and that in the end you need to clean out or replace the gas tank. If there is crude in the shut off valve, then there is probably crude (more than you can drain out without removal) in the tank. What you might find out in a month or two is that bouncing around on those Texas roads is going to continually loosen rusty crap from the inside of the tank. If it has never been done, you should expect that a 50 year old gas tank needs some TLC. There have been a lot of posts about the best way to clean a tank, or whether its better to replace. I cleaned mine. Do a search on the topic and big piles of wisdom will fall on you.
Karl

It may not be the easy way, but it is the Cowboy Way - Ranger Doug
Beware of the stories you read or tell; subtly, at night, beneath the waters of consciousness, they are altering your world. - Ben Okri
1953 Chevy 3100
1960 Volvo PV 544
1941 Chevrolet Special Deluxe