[Southcity]

It's a good question, surprised that you have not received more responses on this. I have a 66 Suburban and had the same difficulty in finding an original style gas cap or just a gas cap that would work. My Suburban came with an aftermarket locking gas cap that never fit well and let fuel spill out very easily. A few things to consider:

1. As I understand it, stock 60-66 Suburban gas tanks do not have a vented line. I believe the original style gas cap would have been vented to help with this, allowing pressure to vent out as well as allow outside air in. I have no idea if these would allow fuel to seep out if the tank is full and driving some curvy roads.
2. There are a few threads that suggest just finding a cap that fits (vented or not) and then drilling a small pin hole in it. This does work but still allows fuel to seep out if tank is full and you drive curvy roads. It only takes a tiny amount of fuel to seep to smell like the entire tank is emptying itself onto your truck or ground.
3. There are some aftermarket caps (i.e Stant) that are vented but only allow pressure to escape, and most times the spring on the valve is a bit too stiff. Some threads talk about taking the cap apart, cutting a few coils from the spring so it is not quite as stiff so the vent opens more easily.

I am currently using a Stant 10640, it is vented but only to allow pressure out. I ended up modifying it by removing the vent spring so pressure is released but also let's air into the tank. The cap fits well but it should be noted I am also using an Edelbrock fuel pump on a 454 BB. I am not sure if the behavior noted above is different with a stock fuel pump/engine. I still get a little bit of fuel seep on it if tank is full and I drive curvy roads, the fuel starts sloshing around, shoots right up the filler neck and finds it's way out anywhere it can. I'm still on the hunt for a better gas cap.