Yes, it makes a difference. You need a tank with a single line. The fuel pump will suck the fuel into the motor. Can you post pictures of the two tanks? Does one of them have a 2 line hose? If so, it is a pressure tank and won’t be appropriate for your motor.
There are a couple different fuel connectors for the single line tanks, and as you say, you may have to swap one of them out. You could take the correct fitting off your 6 gallon tank and put it on the 3 gallon tank with the incorrect fitting.
As for the caps I believe some have vent screws and some do not. If it has a screw it needs to be in the open position when you want to run the motor otherwise as the fuel pump sucks fuel out of the tank it will create a vacuum and fight the fuel pump and your motor won’t get any fuel.
I have only run a couple single line tanks and am not an expert on them. Others will add valuable info here.
Like with any fuel connection, make sure your system is safe and does not leak anywhere along the fuel delivery path.
Scott
