I've just installed my engine mounts and fuel tank on my Kadet Seniorita, I was wondering if I could test my engine by clamping the fuseilage to my workbench? If not what would be the best option. I would like to test it before I close in my fuel tank. I am using copper lines in my tank and I am not sure if my vent line is in properly it seems to want to slip off to the side and not stay up at the top of the tank.

I'm not sure how the vent tube exposes itself outside the tank compartment on that plane, but why not mark the tube to note the orientaion of the vent opening inside the tank? This way you'll know if it rotates later.

Curious, ss there no way to pull the tank later on that kit?

Don't run your engine untill the fusilage is covered and the exposed wood is painted with fuel proof paint. Glow fuel is about 18% oil and most of it comes out of the exhaust and gets all over your plane.
I often check fuel tanks by holding them under water while blowing into the fuel line to pressurize the tank. No air bubbles mean the tank is sealed.

If your vent tube is turning, it sounds like the stopper is not tight enough.

With the design of a Kadet, some guys just build a hatch to allow easy access to the fuel tank. Certainly won't make the front end look much different at all.